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Negotiation failed after updating version.

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by (130 points)

I have an issue where we updated our licensed version of Rebex from 3.0.6586.0 to 5.0.7800.0

after the update we started getting negotiation failed errors.
IE: Negotiation failed. The connection was closed by the server.

If we revert to the older version it works. ultimately we want it to work on the newer version for the security fixes, but will be forced to revert if we cannot solve this problem.

After searching your forum I have tried manipulating the KeyExchangeAlgorithms, PreferredHostKeyAlgorithm, and UseLegacyGroupExchange to see if that would resolve the problem but so far I've been working in the blind.

the code that works with 3.0.6586.0 has these settings:

_client.Settings.SshParameters.KeyExchangeAlgorithms |= SshKeyExchangeAlgorithm.DiffieHellmanGroup1SHA1;
_client.Settings.SshParameters.PreferredHostKeyAlgorithm = SshHostKeyAlgorithm.RSA;

I'll post the failure log in a reply as it is too long.

Applies to: Rebex SFTP
by (130 points)
2021-05-18 16:12:44.298 Opening log file.
2021-05-18 16:12:44.305 INFO FileLogWriter(1)[1] Info: Assembly: Rebex.Common R5.2 for .NET 4.6-4.8
2021-05-18 16:12:44.308 INFO FileLogWriter(1)[1] Info: Platform: Windows 6.2.9200 32-bit; CLR: 4.0.30319.42000
2021-05-18 16:12:44.310 DEBUG FileLogWriter(1)[1] Info: Culture: en; Windows-1252
2021-05-18 16:12:46.436 INFO Sftp(1)[1] Info: Connecting to 207.45.41.162:22 using Sftp.
2021-05-18 16:12:46.436 INFO Sftp(1)[1] Info: Assembly: Rebex.Sftp R5.2 for .NET 4.6-4.8
2021-05-18 16:12:46.438 INFO Sftp(1)[1] Info: Platform: Windows 6.2.9200 32-bit; CLR: 4.0.30319.42000
2021-05-18 16:12:46.438 DEBUG Sftp(1)[1] Info: Culture: en; Windows-1252
2021-05-18 16:12:46.511 DEBUG Sftp(1)[1] Proxy: Connecting to 207.45.41.162:22 (no proxy).
2021-05-18 16:12:46.587 DEBUG Sftp(1)[1] Proxy: Connection established.
2021-05-18 16:12:46.599 VERBOSE Sftp(1)[1] SSH: Sending data:
 0000 |53-53-48-2D-32-2E-30-2D 52-65-62-65-78-53-53-48| SSH-2.0-RebexSSH
 0010 |5F-35-2E-30-2E-37-38-30 30-2E-30-0D-0A         | _5.0.7800.0..
2021-05-18 16:12:46.790 VERBOSE Sftp(1)[1] SSH: Received data:
 0000 |53-53-48-2D-32-2E-30-2D 4F-70-65-6E-53-53-48-5F| SSH-2.0-OpenSSH_
 0010 |37-2E-36-0D-0A                                 | 7.6..
2021-05-18 16:12:46.791 DEBUG Sftp(1)[1] SSH: Server is 'SSH-2.0-OpenSSH_7.6'.
2021-05-18 16:12:46.801 INFO Sftp(1)[1] SSH: Negotiation started.
2021-05-18 16:12:46.816 VERBOSE Sftp(1)[7] SSH: Received packet SSH_MSG_KEXINIT (975 bytes).
 0000 |14-B5-72-05-09-43-1E-09 DB-4C-95-7C-C5-B9-93-3B| ..r..C...L.|...;
 0010 |4A-00-00-01-40-63-75-72 76-65-32-35-35-31-39-2D| J...@curve25519-
 0020 |73-68-61-32-35-36-2C-63 75-72-76-65-32-35-35-31| sha256,curve2551
 0030 |39-2D-73-68-61-32-35-36 40-6C-69-62-73-73-68-2E| 9-sha256@libssh.
 0040 |6F-72-67-2C-65-63-64-68 2D-73-68-61-32-2D-6E-69| org,ecdh-sha2-ni
 0050 |73-74-70-32-35-36-2C-65 63-64-68-2D-73-68-61-32| stp256,ecdh-sha2
 0060 |2D-6E-69-73-74-70-33-38 34-2C-65-63-64-68-2D-73| -nistp384,ecdh-s
 0070 |68-61-32-2D-6E-69-73-74 70-35-32-31-2C-64-69-66| ha2-nistp521,dif
 0080 |66-69-65-2D-68-65-6C-6C 6D-61-6E-2D-67-72-6F-75| fie-hellman-grou
 0090 |70-2D-65-78-63-68-61-6E 67-65-2D-73-68-61-32-35| p-exchange-sha25
 00A0 |36-2C-64-69-66-66-69-65 2D-68-65-6C-6C-6D-61-6E| 6,diffie-hellman
 00B0 |2D-67-72-6F-75-70-31-36 2D-73-68-61-35-31-32-2C| -group16-sha512,
 00C0 |64-69-66-66-69-65-2D-68 65-6C-6C-6D-61-6E-2D-67| diffie-hellman-g
 00D0 |72-6F-75-70-31-38-2D-73 68-61-35-31-32-2C-64-69| roup18-sha512,di
 00E0 |66-66-69-65-2D-68-65-6C 6C-6D-61-6E-2D-67-72-6F| ffie-hellman-gro
 00F0 |75-70-2D-65-78-63-68-61 6E-67-65-2D-73-68-61-31| up-exchange-sha1
 0100 |2C-64-69-66-66-69-65-2D 68-65-6C-6C-6D-61-6E-2D| ,diffie-hellman-
 0110 |67-72-6F-75-70-31-34-2D 73-68-61-32-35-36-2C-64| group14-sha256,d
 0120 |69-66-66-69-65-2D-68-65 6C-6C-6D-61-6E-2D-67-72| iffie-hellman-gr
 0130 |6F-75-70-31-34-2D-73-68 61-31-2C-64-69-66-66-69| oup14-sha1,diffi
 0140 |65-2D-68-65-6C-6C-6D-61 6E-2D-67-72-6F-75-70-31| e-hellman-group1
 0150 |2D-73-68-61-31-00-00-00 21-73-73-68-2D-72-73-61| -sha1...!ssh-rsa
 0160 |2C-72-73-61-2D-73-68-61 32-2D-35-31-32-2C-72-73| ,rsa-sha2-512,rs
 0170 |61-2D-73-68-61-32-2D-32 35-36-00-00-00-6F-61-65| a-sha2-256...oae
 0180 |73-31-32-38-2D-63-74-72 2C-61-65-73-31-39-32-2D| s128-ctr,aes192-
 0190 |63-74-72-2C-61-65-73-32 35-36-2D-63-74-72-2C-61| ctr,aes256-ctr,a
 01A0 |65-73-31-32-38-2D-63-62 63-2C-61-65-73-31-39-32| es128-cbc,aes192
 01B0 |2D-63-62-63-2C-61-65-73 32-35-36-2D-63-62-63-2C| -cbc,aes256-cbc,
 01C0 |61-65-73-32-35-36-2D-67 63-6D-40-6F-70-65-6E-73| aes256-gcm@opens
 01D0 |73-68-2E-63-6F-6D-2C-61 65-73-31-32-38-2D-67-63| sh.com,aes128-gc
 01E0 |6D-40-6F-70-65-6E-73-73 68-2E-63-6F-6D-00-00-00| m@openssh.com...
 01F0 |6F-61-65-73-31-32-38-2D 63-74-72-2C-61-65-73-31| oaes128-ctr,aes1
 0200 |39-32-2D-63-74-72-2C-61 65-73-32-35-36-2D-63-74| 92-ctr,aes256-ct
 0210 |72-2C-61-65-73-31-32-38 2D-63-62-63-2C-61-65-73| r,aes128-cbc,aes
 0220 |31-39-32-2D-63-62-63-2C 61-65-73-32-35-36-2D-63| 192-cbc,aes256-c
 0230 |62-63-2C-61-65-73-32-35 36-2D-67-63-6D-40-6F-70| bc,aes256-gcm@op
 0240 |65-6E-73-73-68-2E-63-6F 6D-2C-61-65-73-31-32-38| enssh.com,aes128
 0250 |2D-67-63-6D-40-6F-70-65 6E-73-73-68-2E-63-6F-6D| -gcm@openssh.com
 0260 |00-00-00-A5-68-6D-61-63 2D-73-68-61-31-2D-65-74| ....hmac-sha1-et
 0270 |6D-40-6F-70-65-6E-73-73 68-2E-63-6F-6D-2C-68-6D| m@openssh.com,hm
 0280 |61-63-2D-73-68-61-32-2D 32-35-36-2D-65-74-6D-40| ac-sha2-256-etm@
 0290 |6F-70-65-6E-73-73-68-2E 63-6F-6D-2C-68-6D-61-63| openssh.com,hmac
 02A0 |2D-73-68-61-32-2D-35-31 32-2D-65-74-6D-40-6F-70| -sha2-512-etm@op
 02B0 |65-6E-73-73-68-2E-63-6F 6D-2C-68-6D-61-63-2D-73| enssh.com,hmac-s
 02C0 |68-61-31-2D-39-36-2D-65 74-6D-40-6F-70-65-6E-73| ha1-96-etm@opens
 02D0 |73-68-2E-63-6F-6D-2C-68 6D-61-63-2D-73-68-61-31| sh.com,hmac-sha1
 02E0 |2C-68-6D-61-63-2D-73-68 61-32-2D-32-35-36-2C-68| ,hmac-sha2-256,h
 02F0 |6D-61-63-2D-73-68-61-32 2D-35-31-32-2C-68-6D-61| mac-sha2-512,hma
 0300 |63-2D-73-68-61-31-2D-39 36-00-00-00-A5-68-6D-61| c-sha1-96....hma
 0310 |63-2D-73-68-61-31-2D-65 74-6D-40-6F-70-65-6E-73| c-sha1-etm@opens
 0320 |73-68-2E-63-6F-6D-2C-68 6D-61-63-2D-73-68-61-32| sh.com,hmac-sha2
 0330 |2D-32-35-36-2D-65-74-6D 40-6F-70-65-6E-73-73-68| -256-etm@openssh
 0340 |2E-63-6F-6D-2C-68-6D-61 63-2D-73-68-61-32-2D-35| .com,hmac-sha2-5
 0350 |31-32-2D-65-74-6D-40-6F 70-65-6E-73-73-68-2E-63| 12-etm@openssh.c
 0360 |6F-6D-2C-68-6D-61-63-2D 73-68-61-31-2D-39-36-2D| om,hmac-sha1-96-
 0370 |65-74-6D-40-6F-70-65-6E 73-73-68-2E-63-6F-6D-2C| etm@openssh.com,
 0380 |68-6D-61-63-2D-73-68-61 31-2C-68-6D-61-63-2D-73| hmac-sha1,hmac-s
 0390 |68-61-32-2D-32-35-36-2C 68-6D-61-63-2D-73-68-61| ha2-256,hmac-sha
 03A0 |32-2D-35-31-32-2C-68-6D 61-63-2D-73-68-61-31-2D| 2-512,hmac-sha1-
 03B0 |39-36-00-00-00-04-6E-6F 6E-65-00-00-00-04-6E-6F| 96....none....no
 03C0 |6E-65-00-00-00-00-00-00 00-00-00-00-00-00-00   | ne.............
by (130 points)
2021-05-18 16:12:46.937 VERBOSE Sftp(1)[1] SSH: Sending packet SSH_MSG_KEXINIT (1282 bytes).
 0000 |14-DA-C5-A4-B2-CB-2F-18 31-C5-0F-5A-65-BF-14-C6| ....../.1..Ze...
 0010 |68-00-00-01-25-63-75-72 76-65-32-35-35-31-39-2D| h...%curve25519-
 0020 |73-68-61-32-35-36-2C-63 75-72-76-65-32-35-35-31| sha256,curve2551
 0030 |39-2D-73-68-61-32-35-36 40-6C-69-62-73-73-68-2E| 9-sha256@libssh.
 0040 |6F-72-67-2C-65-63-64-68 2D-73-68-61-32-2D-6E-69| org,ecdh-sha2-ni
 0050 |73-74-70-32-35-36-2C-64 69-66-66-69-65-2D-68-65| stp256,diffie-he
 0060 |6C-6C-6D-61-6E-2D-67-72 6F-75-70-2D-65-78-63-68| llman-group-exch
 0070 |61-6E-67-65-2D-73-68-61 32-35-36-2C-64-69-66-66| ange-sha256,diff
 0080 |69-65-2D-68-65-6C-6C-6D 61-6E-2D-67-72-6F-75-70| ie-hellman-group
 0090 |31-34-2D-73-68-61-32-35 36-2C-64-69-66-66-69-65| 14-sha256,diffie
 00A0 |2D-68-65-6C-6C-6D-61-6E 2D-67-72-6F-75-70-31-35| -hellman-group15
 00B0 |2D-73-68-61-35-31-32-2C 64-69-66-66-69-65-2D-68| -sha512,diffie-h
 00C0 |65-6C-6C-6D-61-6E-2D-67 72-6F-75-70-31-36-2D-73| ellman-group16-s
 00D0 |68-61-35-31-32-2C-64-69 66-66-69-65-2D-68-65-6C| ha512,diffie-hel
 00E0 |6C-6D-61-6E-2D-67-72-6F 75-70-2D-65-78-63-68-61| lman-group-excha
 00F0 |6E-67-65-2D-73-68-61-31 2C-64-69-66-66-69-65-2D| nge-sha1,diffie-
 0100 |68-65-6C-6C-6D-61-6E-2D 67-72-6F-75-70-31-34-2D| hellman-group14-
 0110 |73-68-61-31-2C-64-69-66 66-69-65-2D-68-65-6C-6C| sha1,diffie-hell
 0120 |6D-61-6E-2D-67-72-6F-75 70-31-2D-73-68-61-31-2C| man-group1-sha1,
 0130 |65-78-74-2D-69-6E-66-6F 2D-63-00-00-00-9F-72-73| ext-info-c....rs
 0140 |61-2D-73-68-61-32-2D-32 35-36-2C-73-73-68-2D-72| a-sha2-256,ssh-r
 0150 |73-61-2D-73-68-61-32-35 36-40-73-73-68-2E-63-6F| sa-sha256@ssh.co
 0160 |6D-2C-72-73-61-2D-73-68 61-32-2D-35-31-32-2C-73| m,rsa-sha2-512,s
 0170 |73-68-2D-72-73-61-2C-73 73-68-2D-64-73-73-2C-78| sh-rsa,ssh-dss,x
 0180 |35-30-39-76-33-2D-73-69 67-6E-2D-72-73-61-2D-73| 509v3-sign-rsa-s
 0190 |68-61-32-35-36-40-73-73 68-2E-63-6F-6D-2C-78-35| ha256@ssh.com,x5
 01A0 |30-39-76-33-2D-73-69-67 6E-2D-72-73-61-2C-78-35| 09v3-sign-rsa,x5
 01B0 |30-39-76-33-2D-73-69-67 6E-2D-64-73-73-2C-73-73| 09v3-sign-dss,ss
 01C0 |68-2D-65-64-32-35-35-31 39-2C-65-63-64-73-61-2D| h-ed25519,ecdsa-
 01D0 |73-68-61-32-2D-6E-69-73 74-70-32-35-36-00-00-01| sha2-nistp256...
 01E0 |05-61-65-73-32-35-36-2D 67-63-6D-40-6F-70-65-6E| .aes256-gcm@open
 01F0 |73-73-68-2E-63-6F-6D-2C 61-65-73-31-32-38-2D-67| ssh.com,aes128-g
 0200 |63-6D-40-6F-70-65-6E-73 73-68-2E-63-6F-6D-2C-61| cm@openssh.com,a
 0210 |65-73-32-35-36-2D-63-74 72-2C-61-65-73-31-39-32| es256-ctr,aes192
 0220 |2D-63-74-72-2C-61-65-73 31-32-38-2D-63-74-72-2C| -ctr,aes128-ctr,
 0230 |33-64-65-73-2D-63-74-72 2C-74-77-6F-66-69-73-68| 3des-ctr,twofish
 0240 |32-35-36-2D-63-74-72-2C 74-77-6F-66-69-73-68-31| 256-ctr,twofish1
 0250 |39-32-2D-63-74-72-2C-74 77-6F-66-69-73-68-31-32| 92-ctr,twofish12
 0260 |38-2D-63-74-72-2C-61-65 73-32-35-36-2D-63-62-63| 8-ctr,aes256-cbc
 0270 |2C-61-65-73-31-39-32-2D 63-62-63-2C-61-65-73-31| ,aes192-cbc,aes1
 0280 |32-38-2D-63-62-63-2C-33 64-65-73-2D-63-62-63-2C| 28-cbc,3des-cbc,
 0290 |74-77-6F-66-69-73-68-32 35-36-2D-63-62-63-2C-74| twofish256-cbc,t
 02A0 |77-6F-66-69-73-68-31-39 32-2D-63-62-63-2C-74-77| wofish192-cbc,tw
 02B0 |6F-66-69-73-68-31-32-38 2D-63-62-63-2C-74-77-6F| ofish128-cbc,two
 02C0 |66-69-73-68-2D-63-62-63 2C-63-68-61-63-68-61-32| fish-cbc,chacha2
 02D0 |30-2D-70-6F-6C-79-31-33 30-35-40-6F-70-65-6E-73| 0-poly1305@opens
 02E0 |73-68-2E-63-6F-6D-00-00 01-05-61-65-73-32-35-36| sh.com....aes256
 02F0 |2D-67-63-6D-40-6F-70-65 6E-73-73-68-2E-63-6F-6D| -gcm@openssh.com
 0300 |2C-61-65-73-31-32-38-2D 67-63-6D-40-6F-70-65-6E| ,aes128-gcm@open
 0310 |73-73-68-2E-63-6F-6D-2C 61-65-73-32-35-36-2D-63| ssh.com,aes256-c
 0320 |74-72-2C-61-65-73-31-39 32-2D-63-74-72-2C-61-65| tr,aes192-ctr,ae
 0330 |73-31-32-38-2D-63-74-72 2C-33-64-65-73-2D-63-74| s128-ctr,3des-ct
 0340 |72-2C-74-77-6F-66-69-73 68-32-35-36-2D-63-74-72| r,twofish256-ctr
 0350 |2C-74-77-6F-66-69-73-68 31-39-32-2D-63-74-72-2C| ,twofish192-ctr,
 0360 |74-77-6F-66-69-73-68-31 32-38-2D-63-74-72-2C-61| twofish128-ctr,a
 0370 |65-73-32-35-36-2D-63-62 63-2C-61-65-73-31-39-32| es256-cbc,aes192
 0380 |2D-63-62-63-2C-61-65-73 31-32-38-2D-63-62-63-2C| -cbc,aes128-cbc,
 0390 |33-64-65-73-2D-63-62-63 2C-74-77-6F-66-69-73-68| 3des-cbc,twofish
 03A0 |32-35-36-2D-63-62-63-2C 74-77-6F-66-69-73-68-31| 256-cbc,twofish1
 03B0 |39-32-2D-63-62-63-2C-74 77-6F-66-69-73-68-31-32| 92-cbc,twofish12
 03C0 |38-2D-63-62-63-2C-74-77 6F-66-69-73-68-2D-63-62| 8-cbc,twofish-cb
 03D0 |63-2C-63-68-61-63-68-61 32-30-2D-70-6F-6C-79-31| c,chacha20-poly1
 03E0 |33-30-35-40-6F-70-65-6E 73-73-68-2E-63-6F-6D-00| 305@openssh.com.
 03F0 |00-00-61-68-6D-61-63-2D 73-68-61-32-2D-32-35-36| ..ahmac-sha2-256
 0400 |2D-65-74-6D-40-6F-70-65 6E-73-73-68-2E-63-6F-6D| -etm@openssh.com
 0410 |2C-68-6D-61-63-2D-73-68 61-32-2D-32-35-36-2C-68| ,hmac-sha2-256,h
 0420 |6D-61-63-2D-73-68-61-32 2D-35-31-32-2D-65-74-6D| mac-sha2-512-etm
 0430 |40-6F-70-65-6E-73-73-68 2E-63-6F-6D-2C-68-6D-61| @openssh.com,hma
 0440 |63-2D-73-68-61-32-2D-35 31-32-2C-68-6D-61-63-2D| c-sha2-512,hmac-
 0450 |73-68-61-31-00-00-00-61 68-6D-61-63-2D-73-68-61| sha1...ahmac-sha
 0460 |32-2D-32-35-36-2D-65-74 6D-40-6F-70-65-6E-73-73| 2-256-etm@openss
 0470 |68-2E-63-6F-6D-2C-68-6D 61-63-2D-73-68-61-32-2D| h.com,hmac-sha2-
 0480 |32-35-36-2C-68-6D-61-63 2D-73-68-61-32-2D-35-31| 256,hmac-sha2-51
 0490 |32-2D-65-74-6D-40-6F-70 65-6E-73-73-68-2E-63-6F| 2-etm@openssh.co
 04A0 |6D-2C-68-6D-61-63-2D-73 68-61-32-2D-35-31-32-2C| m,hmac-sha2-512,
 04B0 |68-6D-61-63-2D-73-68-61 31-00-00-00-1A-6E-6F-6E| hmac-sha1....non
 04C0 |65-2C-7A-6C-69-62-2C-7A 6C-69-62-40-6F-70-65-6E| e,zlib,zlib@open
 04D0 |73-73-68-2E-63-6F-6D-00 00-00-1A-6E-6F-6E-65-2C| ssh.com....none,
 04E0 |7A-6C-69-62-2C-7A-6C-69 62-40-6F-70-65-6E-73-73| zlib,zlib@openss
 04F0 |68-2E-63-6F-6D-00-00-00 00-00-00-00-00-00-00-00| h.com...........
 0500 |00-00                                          | ..
2021-05-18 16:12:46.951 DEBUG Sftp(1)[1] SSH: Negotiating key.
2021-05-18 16:12:46.959 VERBOSE Sftp(1)[1] SSH: Sending packet SSH_MSG_KEX_30 (37 bytes).
 0000 |1E-00-00-00-20-1D-F6-B6 CC-C4-B6-5A-74-70-71-85| .... ......Ztpq.
 0010 |B7-44-8B-14-88-3A-00-3B DB-ED-06-48-6F-98-F3-2E| .D...:.;...Ho...
 0020 |FE-CF-2B-6D-2A                                 | ..+m*
2021-05-18 16:12:47.015 DEBUG Sftp(1)[7] SSH: SSH connection closed.
2021-05-18 16:12:47.020 ERROR Sftp(1)[1] SSH: Negotiation failed. The connection was closed by the server.
2021-05-18 16:12:47.028 ERROR Sftp(1)[1] Info: Rebex.Net.SshException: The connection was closed by the server.
   at Rebex.Net.SshSession.zdtde[T,S](nryaa`2 p0, Int32 p1, wvmhp p2, S p3, T p4, T p5)
   at Rebex.Net.SshSession.yntgu[T,S](nryaa`2 p0, S p1)
   at Rebex.Net.SshSession.fdetz(wjwzv p0)
   at fbmqn.yuozn.acyly(SshSession p0, Byte[] p1, Byte[] p2, Byte[] p3, Byte[] p4, vuqti& p5, Byte[]& p6, SshPublicKey& p7)
   at Rebex.Net.SshSession.dgduw(Byte[] p0)
   at Rebex.Net.SshSession.Negotiate()
   at Rebex.Net.Sftp.fbmqn.mjgdl.gmlbl(rqghl p0, Boolean p1)
   at Rebex.Net.Sftp.yhxos(String p0, Int32 p1, SshParameters p2, rqghl p3)

1 Answer

+1 vote
by (148k points)

This looks like Curve25519 key exchange cipher might be to blame, although it's a bit strange, because we are currently not aware of any incompatibility between Rebex SFTP and OpenSSH 7.6. Please try disabling this cipher and let us know whether that helps:

_client.Settings.SshParameters.KeyExchangeAlgorithms &=
~SshKeyExchangeAlgorithm.Curve25519;

On an unrelated note: unless you really need DiffieHellmanGroup1SHA1 and accept its risks, it's recommended not to enable that cipher because it is considered vulnerable to state-level adversaries.

by (130 points)
After making this change it is still failing:

2021-05-19 12:37:07.148 Opening log file.
2021-05-19 12:37:07.154 INFO FileLogWriter(1)[1] Info: Assembly: Rebex.Common R5.2 for .NET 4.6-4.8
2021-05-19 12:37:07.159 INFO FileLogWriter(1)[1] Info: Platform: Windows 6.2.9200 32-bit; CLR: 4.0.30319.42000
2021-05-19 12:37:07.160 DEBUG FileLogWriter(1)[1] Info: Culture: en; Windows-1252
2021-05-19 12:37:08.941 INFO Sftp(1)[1] Info: Connecting to 207.45.41.162:22 using Sftp.
2021-05-19 12:37:08.942 INFO Sftp(1)[1] Info: Assembly: Rebex.Sftp R5.2 for .NET 4.6-4.8
2021-05-19 12:37:08.943 INFO Sftp(1)[1] Info: Platform: Windows 6.2.9200 32-bit; CLR: 4.0.30319.42000
2021-05-19 12:37:08.943 DEBUG Sftp(1)[1] Info: Culture: en; Windows-1252
2021-05-19 12:37:08.969 DEBUG Sftp(1)[1] Proxy: Connecting to 207.45.41.162:22 (no proxy).
2021-05-19 12:37:18.076 DEBUG Sftp(1)[1] Proxy: Connection established.
2021-05-19 12:37:18.090 VERBOSE Sftp(1)[1] SSH: Sending data:
 0000 |53-53-48-2D-32-2E-30-2D 52-65-62-65-78-53-53-48| SSH-2.0-RebexSSH
 0010 |5F-35-2E-30-2E-37-38-30 30-2E-30-0D-0A         | _5.0.7800.0..
2021-05-19 12:37:18.281 VERBOSE Sftp(1)[1] SSH: Received data:
 0000 |53-53-48-2D-32-2E-30-2D 4F-70-65-6E-53-53-48-5F| SSH-2.0-OpenSSH_
 0010 |37-2E-36-0D-0A                                 | 7.6..
2021-05-19 12:37:18.282 DEBUG Sftp(1)[1] SSH: Server is 'SSH-2.0-OpenSSH_7.6'.
2021-05-19 12:37:18.300 INFO Sftp(1)[1] SSH: Negotiation started.
2021-05-19 12:37:18.323 VERBOSE Sftp(1)[6] SSH: Received packet SSH_MSG_KEXINIT (975 bytes).
 0000 |14-95-2E-9D-0E-67-A4-90 3A-67-48-27-1E-F4-CB-46| .....g..:gH'...F
 0010 |36-00-00-01-40-63-75-72 76-65-32-35-35-31-39-2D| 6...@curve25519-
 0020 |73-68-61-32-35-36-2C-63 75-72-76-65-32-35-35-31| sha256,curve2551
 0030 |39-2D-73-68-61-32-35-36 40-6C-69-62-73-73-68-2E| 9-sha256@libssh.
 0040 |6F-72-67-2C-65-63-64-68 2D-73-68-61-32-2D-6E-69| org,ecdh-sha2-ni
 0050 |73-74-70-32-35-36-2C-65 63-64-68-2D-73-68-61-32| stp256,ecdh-sha2
 0060 |2D-6E-69-73-74-70-33-38 34-2C-65-63-64-68-2D-73| -nistp384,ecdh-s
 0070 |68-61-32-2D-6E-69-73-74 70-35-32-31-2C-64-69-66| ha2-nistp521,dif
 0080 |66-69-65-2D-68-65-6C-6C 6D-61-6E-2D-67-72-6F-75| fie-hellman-grou
 0090 |70-2D-65-78-63-68-61-6E 67-65-2D-73-68-61-32-35| p-exchange-sha25
 00A0 |36-2C-64-69-66-66-69-65 2D-68-65-6C-6C-6D-61-6E| 6,diffie-hellman
 00B0 |2D-67-72-6F-75-70-31-36 2D-73-68-61-35-31-32-2C| -group16-sha512,
 00C0 |64-69-66-66-69-65-2D-68 65-6C-6C-6D-61-6E-2D-67| diffie-hellman-g
 00D0 |72-6F-75-70-31-38-2D-73 68-61-35-31-32-2C-64-69| roup18-sha512,di
 00E0 |66-66-69-65-2D-68-65-6C 6C-6D-61-6E-2D-67-72-6F| ffie-hellman-gro
 00F0 |75-70-2D-65-78-63-68-61 6E-67-65-2D-73-68-61-31| up-exchange-sha1
 0100 |2C-64-69-66-66-69-65-2D 68-65-6C-6C-6D-61-6E-2D| ,diffie-hellman-
 0110 |67-72-6F-75-70-31-34-2D 73-68-61-32-35-36-2C-64| group14-sha256,d
 0120 |69-66-66-69-65-2D-68-65 6C-6C-6D-61-6E-2D-67-72| iffie-hellman-gr
 0130 |6F-75-70-31-34-2D-73-68 61-31-2C-64-69-66-66-69| oup14-sha1,diffi
 0140 |65-2D-68-65-6C-6C-6D-61 6E-2D-67-72-6F-75-70-31| e-hellman-group1
 0150 |2D-73-68-61-31-00-00-00 21-73-73-68-2D-72-73-61| -sha1...!ssh-rsa
 0160 |2C-72-73-61-2D-73-68-61 32-2D-35-31-32-2C-72-73| ,rsa-sha2-512,rs
 0170 |61-2D-73-68-61-32-2D-32 35-36-00-00-00-6F-61-65| a-sha2-256...oae
 0180 |73-31-32-38-2D-63-74-72 2C-61-65-73-31-39-32-2D| s128-ctr,aes192-
 0190 |63-74-72-2C-61-65-73-32 35-36-2D-63-74-72-2C-61| ctr,aes256-ctr,a
 01A0 |65-73-31-32-38-2D-63-62 63-2C-61-65-73-31-39-32| es128-cbc,aes192
 01B0 |2D-63-62-63-2C-61-65-73 32-35-36-2D-63-62-63-2C| -cbc,aes256-cbc,
 01C0 |61-65-73-32-35-36-2D-67 63-6D-40-6F-70-65-6E-73| aes256-gcm@opens
 01D0 |73-68-2E-63-6F-6D-2C-61 65-73-31-32-38-2D-67-63| sh.com,aes128-gc
 01E0 |6D-40-6F-70-65-6E-73-73 68-2E-63-6F-6D-00-00-00| m@openssh.com...
 01F0 |6F-61-65-73-31-32-38-2D 63-74-72-2C-61-65-73-31| oaes128-ctr,aes1
 0200 |39-32-2D-63-74-72-2C-61 65-73-32-35-36-2D-63-74| 92-ctr,aes256-ct
 0210 |72-2C-61-65-73-31-32-38 2D-63-62-63-2C-61-65-73| r,aes128-cbc,aes
 0220 |31-39-32-2D-63-62-63-2C 61-65-73-32-35-36-2D-63| 192-cbc,aes256-c
 0230 |62-63-2C-61-65-73-32-35 36-2D-67-63-6D-40-6F-70| bc,aes256-gcm@op
 0240 |65-6E-73-73-68-2E-63-6F 6D-2C-61-65-73-31-32-38| enssh.com,aes128
 0250 |2D-67-63-6D-40-6F-70-65 6E-73-73-68-2E-63-6F-6D| -gcm@openssh.com
 0260 |00-00-00-A5-68-6D-61-63 2D-73-68-61-31-2D-65-74| ....hmac-sha1-et
 0270 |6D-40-6F-70-65-6E-73-73 68-2E-63-6F-6D-2C-68-6D| m@openssh.com,hm
 0280 |61-63-2D-73-68-61-32-2D 32-35-36-2D-65-74-6D-40| ac-sha2-256-etm@
 0290 |6F-70-65-6E-73-73-68-2E 63-6F-6D-2C-68-6D-61-63| openssh.com,hmac
 02A0 |2D-73-68-61-32-2D-35-31 32-2D-65-74-6D-40-6F-70| -sha2-512-etm@op
 02B0 |65-6E-73-73-68-2E-63-6F 6D-2C-68-6D-61-63-2D-73| enssh.com,hmac-s
 02C0 |68-61-31-2D-39-36-2D-65 74-6D-40-6F-70-65-6E-73| ha1-96-etm@opens
 02D0 |73-68-2E-63-6F-6D-2C-68 6D-61-63-2D-73-68-61-31| sh.com,hmac-sha1
 02E0 |2C-68-6D-61-63-2D-73-68 61-32-2D-32-35-36-2C-68| ,hmac-sha2-256,h
 02F0 |6D-61-63-2D-73-68-61-32 2D-35-31-32-2C-68-6D-61| mac-sha2-512,hma
 0300 |63-2D-73-68-61-31-2D-39 36-00-00-00-A5-68-6D-61| c-sha1-96....hma
 0310 |63-2D-73-68-61-31-2D-65 74-6D-40-6F-70-65-6E-73| c-sha1-etm@opens
 0320 |73-68-2E-63-6F-6D-2C-68 6D-61-63-2D-73-68-61-32| sh.com,hmac-sha2
 0330 |2D-32-35-36-2D-65-74-6D 40-6F-70-65-6E-73-73-68| -256-etm@openssh
 0340 |2E-63-6F-6D-2C-68-6D-61 63-2D-73-68-61-32-2D-35| .com,hmac-sha2-5
 0350 |31-32-2D-65-74-6D-40-6F 70-65-6E-73-73-68-2E-63| 12-etm@openssh.c
 0360 |6F-6D-2C-68-6D-61-63-2D 73-68-61-31-2D-39-36-2D| om,hmac-sha1-96-
 0370 |65-74-6D-40-6F-70-65-6E 73-73-68-2E-63-6F-6D-2C| etm@openssh.com,
 0380 |68-6D-61-63-2D-73-68-61 31-2C-68-6D-61-63-2D-73| hmac-sha1,hmac-s
 0390 |68-61-32-2D-32-35-36-2C 68-6D-61-63-2D-73-68-61| ha2-256,hmac-sha
 03A0 |32-2D-35-31-32-2C-68-6D 61-63-2D-73-68-61-31-2D| 2-512,hmac-sha1-
 03B0 |39-36-00-00-00-04-6E-6F 6E-65-00-00-00-04-6E-6F| 96....none....no
 03C0 |6E-65-00-00-00-00-00-00 00-00-00-00-00-00-00   | ne.............
by (130 points)
2021-05-19 12:37:18.460 VERBOSE Sftp(1)[1] SSH: Sending packet SSH_MSG_KEXINIT (1273 bytes).
 0000 |14-AD-61-1A-6F-BC-42-20 63-5F-1D-8E-89-AD-7B-E9| ..a.o.B c_....{.
 0010 |24-00-00-01-1C-65-63-64 68-2D-73-68-61-32-2D-6E| $....ecdh-sha2-n
 0020 |69-73-74-70-32-35-36-2C 65-63-64-68-2D-73-68-61| istp256,ecdh-sha
 0030 |32-2D-6E-69-73-74-70-33 38-34-2C-65-63-64-68-2D| 2-nistp384,ecdh-
 0040 |73-68-61-32-2D-6E-69-73 74-70-35-32-31-2C-64-69| sha2-nistp521,di
 0050 |66-66-69-65-2D-68-65-6C 6C-6D-61-6E-2D-67-72-6F| ffie-hellman-gro
 0060 |75-70-2D-65-78-63-68-61 6E-67-65-2D-73-68-61-32| up-exchange-sha2
 0070 |35-36-2C-64-69-66-66-69 65-2D-68-65-6C-6C-6D-61| 56,diffie-hellma
 0080 |6E-2D-67-72-6F-75-70-31 34-2D-73-68-61-32-35-36| n-group14-sha256
 0090 |2C-64-69-66-66-69-65-2D 68-65-6C-6C-6D-61-6E-2D| ,diffie-hellman-
 00A0 |67-72-6F-75-70-31-35-2D 73-68-61-35-31-32-2C-64| group15-sha512,d
 00B0 |69-66-66-69-65-2D-68-65 6C-6C-6D-61-6E-2D-67-72| iffie-hellman-gr
 00C0 |6F-75-70-31-36-2D-73-68 61-35-31-32-2C-64-69-66| oup16-sha512,dif
 00D0 |66-69-65-2D-68-65-6C-6C 6D-61-6E-2D-67-72-6F-75| fie-hellman-grou
 00E0 |70-2D-65-78-63-68-61-6E 67-65-2D-73-68-61-31-2C| p-exchange-sha1,
 00F0 |64-69-66-66-69-65-2D-68 65-6C-6C-6D-61-6E-2D-67| diffie-hellman-g
 0100 |72-6F-75-70-31-34-2D-73 68-61-31-2C-64-69-66-66| roup14-sha1,diff
 0110 |69-65-2D-68-65-6C-6C-6D 61-6E-2D-67-72-6F-75-70| ie-hellman-group
 0120 |31-2D-73-68-61-31-2C-65 78-74-2D-69-6E-66-6F-2D| 1-sha1,ext-info-
 0130 |63-00-00-00-9F-72-73-61 2D-73-68-61-32-2D-32-35| c....rsa-sha2-25
 0140 |36-2C-73-73-68-2D-72-73 61-2D-73-68-61-32-35-36| 6,ssh-rsa-sha256
 0150 |40-73-73-68-2E-63-6F-6D 2C-72-73-61-2D-73-68-61| @ssh.com,rsa-sha
 0160 |32-2D-35-31-32-2C-73-73 68-2D-72-73-61-2C-73-73| 2-512,ssh-rsa,ss
 0170 |68-2D-64-73-73-2C-78-35 30-39-76-33-2D-73-69-67| h-dss,x509v3-sig
 0180 |6E-2D-72-73-61-2D-73-68 61-32-35-36-40-73-73-68| n-rsa-sha256@ssh
 0190 |2E-63-6F-6D-2C-78-35-30 39-76-33-2D-73-69-67-6E| .com,x509v3-sign
 01A0 |2D-72-73-61-2C-78-35-30 39-76-33-2D-73-69-67-6E| -rsa,x509v3-sign
 01B0 |2D-64-73-73-2C-73-73-68 2D-65-64-32-35-35-31-39| -dss,ssh-ed25519
 01C0 |2C-65-63-64-73-61-2D-73 68-61-32-2D-6E-69-73-74| ,ecdsa-sha2-nist
 01D0 |70-32-35-36-00-00-01-05 61-65-73-32-35-36-2D-67| p256....aes256-g
 01E0 |63-6D-40-6F-70-65-6E-73 73-68-2E-63-6F-6D-2C-61| cm@openssh.com,a
 01F0 |65-73-31-32-38-2D-67-63 6D-40-6F-70-65-6E-73-73| es128-gcm@openss
 0200 |68-2E-63-6F-6D-2C-61-65 73-32-35-36-2D-63-74-72| h.com,aes256-ctr
 0210 |2C-61-65-73-31-39-32-2D 63-74-72-2C-61-65-73-31| ,aes192-ctr,aes1
 0220 |32-38-2D-63-74-72-2C-33 64-65-73-2D-63-74-72-2C| 28-ctr,3des-ctr,
 0230 |74-77-6F-66-69-73-68-32 35-36-2D-63-74-72-2C-74| twofish256-ctr,t
 0240 |77-6F-66-69-73-68-31-39 32-2D-63-74-72-2C-74-77| wofish192-ctr,tw
 0250 |6F-66-69-73-68-31-32-38 2D-63-74-72-2C-61-65-73| ofish128-ctr,aes
 0260 |32-35-36-2D-63-62-63-2C 61-65-73-31-39-32-2D-63| 256-cbc,aes192-c
 0270 |62-63-2C-61-65-73-31-32 38-2D-63-62-63-2C-33-64| bc,aes128-cbc,3d
 0280 |65-73-2D-63-62-63-2C-74 77-6F-66-69-73-68-32-35| es-cbc,twofish25
 0290 |36-2D-63-62-63-2C-74-77 6F-66-69-73-68-31-39-32| 6-cbc,twofish192
 02A0 |2D-63-62-63-2C-74-77-6F 66-69-73-68-31-32-38-2D| -cbc,twofish128-
 02B0 |63-62-63-2C-74-77-6F-66 69-73-68-2D-63-62-63-2C| cbc,twofish-cbc,
 02C0 |63-68-61-63-68-61-32-30 2D-70-6F-6C-79-31-33-30| chacha20-poly130
 02D0 |35-40-6F-70-65-6E-73-73 68-2E-63-6F-6D-00-00-01| 5@openssh.com...
 02E0 |05-61-65-73-32-35-36-2D 67-63-6D-40-6F-70-65-6E| .aes256-gcm@open
 02F0 |73-73-68-2E-63-6F-6D-2C 61-65-73-31-32-38-2D-67| ssh.com,aes128-g
 0300 |63-6D-40-6F-70-65-6E-73 73-68-2E-63-6F-6D-2C-61| cm@openssh.com,a
 0310 |65-73-32-35-36-2D-63-74 72-2C-61-65-73-31-39-32| es256-ctr,aes192
 0320 |2D-63-74-72-2C-61-65-73 31-32-38-2D-63-74-72-2C| -ctr,aes128-ctr,
 0330 |33-64-65-73-2D-63-74-72 2C-74-77-6F-66-69-73-68| 3des-ctr,twofish
 0340 |32-35-36-2D-63-74-72-2C 74-77-6F-66-69-73-68-31| 256-ctr,twofish1
 0350 |39-32-2D-63-74-72-2C-74 77-6F-66-69-73-68-31-32| 92-ctr,twofish12
 0360 |38-2D-63-74-72-2C-61-65 73-32-35-36-2D-63-62-63| 8-ctr,aes256-cbc
 0370 |2C-61-65-73-31-39-32-2D 63-62-63-2C-61-65-73-31| ,aes192-cbc,aes1
 0380 |32-38-2D-63-62-63-2C-33 64-65-73-2D-63-62-63-2C| 28-cbc,3des-cbc,
 0390 |74-77-6F-66-69-73-68-32 35-36-2D-63-62-63-2C-74| twofish256-cbc,t
 03A0 |77-6F-66-69-73-68-31-39 32-2D-63-62-63-2C-74-77| wofish192-cbc,tw
 03B0 |6F-66-69-73-68-31-32-38 2D-63-62-63-2C-74-77-6F| ofish128-cbc,two
 03C0 |66-69-73-68-2D-63-62-63 2C-63-68-61-63-68-61-32| fish-cbc,chacha2
 03D0 |30-2D-70-6F-6C-79-31-33 30-35-40-6F-70-65-6E-73| 0-poly1305@opens
 03E0 |73-68-2E-63-6F-6D-00-00 00-61-68-6D-61-63-2D-73| sh.com...ahmac-s
 03F0 |68-61-32-2D-32-35-36-2D 65-74-6D-40-6F-70-65-6E| ha2-256-etm@open
 0400 |73-73-68-2E-63-6F-6D-2C 68-6D-61-63-2D-73-68-61| ssh.com,hmac-sha
 0410 |32-2D-32-35-36-2C-68-6D 61-63-2D-73-68-61-32-2D| 2-256,hmac-sha2-
 0420 |35-31-32-2D-65-74-6D-40 6F-70-65-6E-73-73-68-2E| 512-etm@openssh.
 0430 |63-6F-6D-2C-68-6D-61-63 2D-73-68-61-32-2D-35-31| com,hmac-sha2-51
 0440 |32-2C-68-6D-61-63-2D-73 68-61-31-00-00-00-61-68| 2,hmac-sha1...ah
 0450 |6D-61-63-2D-73-68-61-32 2D-32-35-36-2D-65-74-6D| mac-sha2-256-etm
 0460 |40-6F-70-65-6E-73-73-68 2E-63-6F-6D-2C-68-6D-61| @openssh.com,hma
 0470 |63-2D-73-68-61-32-2D-32 35-36-2C-68-6D-61-63-2D| c-sha2-256,hmac-
 0480 |73-68-61-32-2D-35-31-32 2D-65-74-6D-40-6F-70-65| sha2-512-etm@ope
 0490 |6E-73-73-68-2E-63-6F-6D 2C-68-6D-61-63-2D-73-68| nssh.com,hmac-sh
 04A0 |61-32-2D-35-31-32-2C-68 6D-61-63-2D-73-68-61-31| a2-512,hmac-sha1
 04B0 |00-00-00-1A-6E-6F-6E-65 2C-7A-6C-69-62-2C-7A-6C| ....none,zlib,zl
 04C0 |69-62-40-6F-70-65-6E-73 73-68-2E-63-6F-6D-00-00| ib@openssh.com..
 04D0 |00-1A-6E-6F-6E-65-2C-7A 6C-69-62-2C-7A-6C-69-62| ..none,zlib,zlib
 04E0 |40-6F-70-65-6E-73-73-68 2E-63-6F-6D-00-00-00-00| @openssh.com....
 04F0 |00-00-00-00-00-00-00-00 00                     | .........
2021-05-19 12:37:18.477 DEBUG Sftp(1)[1] SSH: Negotiating key.
2021-05-19 12:37:18.493 VERBOSE Sftp(1)[1] SSH: Sending packet SSH_MSG_KEX_30 (70 bytes).
 0000 |1E-00-00-00-41-04-ED-3E 0F-D1-E8-E8-15-D9-EA-46| ....A..>.......F
 0010 |E5-65-08-1A-5D-2E-02-87 17-39-AD-4A-EB-B7-75-7E| .e..]....9.J..u~
 0020 |D7-75-00-02-C8-26-C3-69 2C-6C-DD-13-BF-9B-FB-CD| .u...&.i,l......
 0030 |B3-74-2D-78-A2-AD-96-A8 51-4D-F2-CF-FE-24-E8-91| .t-x....QM...$..
 0040 |8E-54-8C-60-56-EF                              | .T.`V.
2021-05-19 12:37:18.538 DEBUG Sftp(1)[6] SSH: SSH connection closed.
2021-05-19 12:37:18.544 ERROR Sftp(1)[1] SSH: Negotiation failed. The connection was closed by the server.
2021-05-19 12:37:18.560 ERROR Sftp(1)[1] Info: Rebex.Net.SshException: The connection was closed by the server.
   at Rebex.Net.SshSession.zdtde[T,S](nryaa`2 p0, Int32 p1, wvmhp p2, S p3, T p4, T p5)
   at Rebex.Net.SshSession.yntgu[T,S](nryaa`2 p0, S p1)
   at Rebex.Net.SshSession.fdetz(wjwzv p0)
   at fbmqn.yuozn.acyly(SshSession p0, Byte[] p1, Byte[] p2, Byte[] p3, Byte[] p4, vuqti& p5, Byte[]& p6, SshPublicKey& p7)
   at Rebex.Net.SshSession.dgduw(Byte[] p0)
   at Rebex.Net.SshSession.Negotiate()
   at Rebex.Net.Sftp.fbmqn.mjgdl.gmlbl(rqghl p0, Boolean p1)
   at Rebex.Net.Sftp.yhxos(String p0, Int32 p1, SshParameters p2, rqghl p3)
by (148k points)
Thanks for the update, looks like Curve25519 might not be to blame. Could you please try disabling NIST-P ECDH curves as well?

_client.Settings.SshParameters.KeyExchangeAlgorithms &=
~(SshKeyExchangeAlgorithm.Curve25519 | SshKeyExchangeAlgorithm.ECDiffieHellmanNistP256 | SshKeyExchangeAlgorithm.ECDiffieHellmanNistP384 | SshKeyExchangeAlgorithm.ECDiffieHellmanNistP521);

If that still fails, please post a log created with v3.0.6586.0 for comparison, that would hopefully provide some clues.
by (130 points)
it still failed.
Here is the success log with the older version:

2021-05-19 12:54:16.689 Opening log file.
2021-05-19 12:54:16.695 INFO FileLogWriter(1)[1] Info: Assembly: Rebex.Common 2017 R6.3 for .NET 4.0-4.7
2021-05-19 12:54:16.698 INFO FileLogWriter(1)[1] Info: Platform: Windows 6.2.9200 32-bit; CLR: 4.0.30319.42000
2021-05-19 12:54:16.699 DEBUG FileLogWriter(1)[1] Info: Culture: en; Windows-1252
2021-05-19 12:54:18.832 INFO Sftp(1)[1] Info: Connecting to 207.45.41.162:22 using Sftp.
2021-05-19 12:54:18.833 INFO Sftp(1)[1] Info: Assembly: Rebex.Sftp 2017 R6.3 for .NET 4.0-4.7
2021-05-19 12:54:18.834 INFO Sftp(1)[1] Info: Platform: Windows 6.2.9200 32-bit; CLR: 4.0.30319.42000
2021-05-19 12:54:18.834 DEBUG Sftp(1)[1] Info: Culture: en; Windows-1252
2021-05-19 12:54:18.880 DEBUG ProxySocket(1)[1] Proxy: Connecting to none proxy at 207.45.41.162:22.
2021-05-19 12:54:28.142 VERBOSE Sftp(1)[1] SSH: Sending data:
 0000 |53-53-48-2D-32-2E-30-2D 52-65-62-65-78-53-53-48| SSH-2.0-RebexSSH
 0010 |5F-33-2E-30-2E-36-35-38 36-2E-30-0D-0A         | _3.0.6586.0..
2021-05-19 12:54:28.244 VERBOSE Sftp(1)[1] SSH: Received data:
 0000 |53-53-48-2D-32-2E-30-2D 4F-70-65-6E-53-53-48-5F| SSH-2.0-OpenSSH_
 0010 |37-2E-36-0D-0A                                 | 7.6..
2021-05-19 12:54:28.245 DEBUG Sftp(1)[1] SSH: Server is 'SSH-2.0-OpenSSH_7.6'.
2021-05-19 12:54:28.258 INFO Sftp(1)[1] SSH: Negotiation started.
2021-05-19 12:54:28.278 VERBOSE Sftp(1)[6] SSH: Received packet SSH_MSG_KEXINIT (975 bytes).
 0000 |14-17-DD-40-A3-7B-47-2C 49-26-1C-46-A3-96-86-13| ...@.{G,I&.F....
 0010 |E8-00-00-01-40-63-75-72 76-65-32-35-35-31-39-2D| ....@curve25519-
 0020 |73-68-61-32-35-36-2C-63 75-72-76-65-32-35-35-31| sha256,curve2551
 0030 |39-2D-73-68-61-32-35-36 40-6C-69-62-73-73-68-2E| 9-sha256@libssh.
 0040 |6F-72-67-2C-65-63-64-68 2D-73-68-61-32-2D-6E-69| org,ecdh-sha2-ni
 0050 |73-74-70-32-35-36-2C-65 63-64-68-2D-73-68-61-32| stp256,ecdh-sha2
 0060 |2D-6E-69-73-74-70-33-38 34-2C-65-63-64-68-2D-73| -nistp384,ecdh-s
 0070 |68-61-32-2D-6E-69-73-74 70-35-32-31-2C-64-69-66| ha2-nistp521,dif
 0080 |66-69-65-2D-68-65-6C-6C 6D-61-6E-2D-67-72-6F-75| fie-hellman-grou
 0090 |70-2D-65-78-63-68-61-6E 67-65-2D-73-68-61-32-35| p-exchange-sha25
 00A0 |36-2C-64-69-66-66-69-65 2D-68-65-6C-6C-6D-61-6E| 6,diffie-hellman
 00B0 |2D-67-72-6F-75-70-31-36 2D-73-68-61-35-31-32-2C| -group16-sha512,
 00C0 |64-69-66-66-69-65-2D-68 65-6C-6C-6D-61-6E-2D-67| diffie-hellman-g
 00D0 |72-6F-75-70-31-38-2D-73 68-61-35-31-32-2C-64-69| roup18-sha512,di
 00E0 |66-66-69-65-2D-68-65-6C 6C-6D-61-6E-2D-67-72-6F| ffie-hellman-gro
 00F0 |75-70-2D-65-78-63-68-61 6E-67-65-2D-73-68-61-31| up-exchange-sha1
 0100 |2C-64-69-66-66-69-65-2D 68-65-6C-6C-6D-61-6E-2D| ,diffie-hellman-
 0110 |67-72-6F-75-70-31-34-2D 73-68-61-32-35-36-2C-64| group14-sha256,d
 0120 |69-66-66-69-65-2D-68-65 6C-6C-6D-61-6E-2D-67-72| iffie-hellman-gr
 0130 |6F-75-70-31-34-2D-73-68 61-31-2C-64-69-66-66-69| oup14-sha1,diffi
 0140 |65-2D-68-65-6C-6C-6D-61 6E-2D-67-72-6F-75-70-31| e-hellman-group1
 0150 |2D-73-68-61-31-00-00-00 21-73-73-68-2D-72-73-61| -sha1...!ssh-rsa
 0160 |2C-72-73-61-2D-73-68-61 32-2D-35-31-32-2C-72-73| ,rsa-sha2-512,rs
 0170 |61-2D-73-68-61-32-2D-32 35-36-00-00-00-6F-61-65| a-sha2-256...oae
 0180 |73-31-32-38-2D-63-74-72 2C-61-65-73-31-39-32-2D| s128-ctr,aes192-
 0190 |63-74-72-2C-61-65-73-32 35-36-2D-63-74-72-2C-61| ctr,aes256-ctr,a
 01A0 |65-73-31-32-38-2D-63-62 63-2C-61-65-73-31-39-32| es128-cbc,aes192
 01B0 |2D-63-62-63-2C-61-65-73 32-35-36-2D-63-62-63-2C| -cbc,aes256-cbc,
 01C0 |61-65-73-32-35-36-2D-67 63-6D-40-6F-70-65-6E-73| aes256-gcm@opens
 01D0 |73-68-2E-63-6F-6D-2C-61 65-73-31-32-38-2D-67-63| sh.com,aes128-gc
 01E0 |6D-40-6F-70-65-6E-73-73 68-2E-63-6F-6D-00-00-00| m@openssh.com...
 01F0 |6F-61-65-73-31-32-38-2D 63-74-72-2C-61-65-73-31| oaes128-ctr,aes1
 0200 |39-32-2D-63-74-72-2C-61 65-73-32-35-36-2D-63-74| 92-ctr,aes256-ct
 0210 |72-2C-61-65-73-31-32-38 2D-63-62-63-2C-61-65-73| r,aes128-cbc,aes
 0220 |31-39-32-2D-63-62-63-2C 61-65-73-32-35-36-2D-63| 192-cbc,aes256-c
 0230 |62-63-2C-61-65-73-32-35 36-2D-67-63-6D-40-6F-70| bc,aes256-gcm@op
 0240 |65-6E-73-73-68-2E-63-6F 6D-2C-61-65-73-31-32-38| enssh.com,aes128
 0250 |2D-67-63-6D-40-6F-70-65 6E-73-73-68-2E-63-6F-6D| -gcm@openssh.com
 0260 |00-00-00-A5-68-6D-61-63 2D-73-68-61-31-2D-65-74| ....hmac-sha1-et
 0270 |6D-40-6F-70-65-6E-73-73 68-2E-63-6F-6D-2C-68-6D| m@openssh.com,hm
 0280 |61-63-2D-73-68-61-32-2D 32-35-36-2D-65-74-6D-40| ac-sha2-256-etm@
 0290 |6F-70-65-6E-73-73-68-2E 63-6F-6D-2C-68-6D-61-63| openssh.com,hmac
 02A0 |2D-73-68-61-32-2D-35-31 32-2D-65-74-6D-40-6F-70| -sha2-512-etm@op
 02B0 |65-6E-73-73-68-2E-63-6F 6D-2C-68-6D-61-63-2D-73| enssh.com,hmac-s
 02C0 |68-61-31-2D-39-36-2D-65 74-6D-40-6F-70-65-6E-73| ha1-96-etm@opens
 02D0 |73-68-2E-63-6F-6D-2C-68 6D-61-63-2D-73-68-61-31| sh.com,hmac-sha1
 02E0 |2C-68-6D-61-63-2D-73-68 61-32-2D-32-35-36-2C-68| ,hmac-sha2-256,h
 02F0 |6D-61-63-2D-73-68-61-32 2D-35-31-32-2C-68-6D-61| mac-sha2-512,hma
 0300 |63-2D-73-68-61-31-2D-39 36-00-00-00-A5-68-6D-61| c-sha1-96....hma
 0310 |63-2D-73-68-61-31-2D-65 74-6D-40-6F-70-65-6E-73| c-sha1-etm@opens
 0320 |73-68-2E-63-6F-6D-2C-68 6D-61-63-2D-73-68-61-32| sh.com,hmac-sha2
 0330 |2D-32-35-36-2D-65-74-6D 40-6F-70-65-6E-73-73-68| -256-etm@openssh
 0340 |2E-63-6F-6D-2C-68-6D-61 63-2D-73-68-61-32-2D-35| .com,hmac-sha2-5
 0350 |31-32-2D-65-74-6D-40-6F 70-65-6E-73-73-68-2E-63| 12-etm@openssh.c
 0360 |6F-6D-2C-68-6D-61-63-2D 73-68-61-31-2D-39-36-2D| om,hmac-sha1-96-
 0370 |65-74-6D-40-6F-70-65-6E 73-73-68-2E-63-6F-6D-2C| etm@openssh.com,
 0380 |68-6D-61-63-2D-73-68-61 31-2C-68-6D-61-63-2D-73| hmac-sha1,hmac-s
 0390 |68-61-32-2D-32-35-36-2C 68-6D-61-63-2D-73-68-61| ha2-256,hmac-sha
 03A0 |32-2D-35-31-32-2C-68-6D 61-63-2D-73-68-61-31-2D| 2-512,hmac-sha1-
 03B0 |39-36-00-00-00-04-6E-6F 6E-65-00-00-00-04-6E-6F| 96....none....no
 03C0 |6E-65-00-00-00-00-00-00 00-00-00-00-00-00-00   | ne.............
by (130 points)
2021-05-19 12:54:28.355 VERBOSE Sftp(1)[1] SSH: Sending packet SSH_MSG_KEXINIT (940 bytes).
 0000 |14-5B-FD-C1-3F-CB-13-DA B7-71-A5-83-66-CD-73-F4| .[..?....q..f.s.
 0010 |84-00-00-00-EB-64-69-66 66-69-65-2D-68-65-6C-6C| .....diffie-hell
 0020 |6D-61-6E-2D-67-72-6F-75 70-2D-65-78-63-68-61-6E| man-group-exchan
 0030 |67-65-2D-73-68-61-32-35 36-2C-64-69-66-66-69-65| ge-sha256,diffie
 0040 |2D-68-65-6C-6C-6D-61-6E 2D-67-72-6F-75-70-31-34| -hellman-group14
 0050 |2D-73-68-61-32-35-36-2C 64-69-66-66-69-65-2D-68| -sha256,diffie-h
 0060 |65-6C-6C-6D-61-6E-2D-67 72-6F-75-70-31-35-2D-73| ellman-group15-s
 0070 |68-61-35-31-32-2C-64-69 66-66-69-65-2D-68-65-6C| ha512,diffie-hel
 0080 |6C-6D-61-6E-2D-67-72-6F 75-70-31-36-2D-73-68-61| lman-group16-sha
 0090 |35-31-32-2C-64-69-66-66 69-65-2D-68-65-6C-6C-6D| 512,diffie-hellm
 00A0 |61-6E-2D-67-72-6F-75-70 2D-65-78-63-68-61-6E-67| an-group-exchang
 00B0 |65-2D-73-68-61-31-2C-64 69-66-66-69-65-2D-68-65| e-sha1,diffie-he
 00C0 |6C-6C-6D-61-6E-2D-67-72 6F-75-70-31-34-2D-73-68| llman-group14-sh
 00D0 |61-31-2C-65-63-64-68-2D 73-68-61-32-2D-6E-69-73| a1,ecdh-sha2-nis
 00E0 |74-70-32-35-36-2C-64-69 66-66-69-65-2D-68-65-6C| tp256,diffie-hel
 00F0 |6C-6D-61-6E-2D-67-72-6F 75-70-31-2D-73-68-61-31| lman-group1-sha1
 0100 |00-00-00-93-72-73-61-2D 73-68-61-32-2D-32-35-36| ....rsa-sha2-256
 0110 |2C-73-73-68-2D-72-73-61 2D-73-68-61-32-35-36-40| ,ssh-rsa-sha256@
 0120 |73-73-68-2E-63-6F-6D-2C 72-73-61-2D-73-68-61-32| ssh.com,rsa-sha2
 0130 |2D-35-31-32-2C-73-73-68 2D-72-73-61-2C-73-73-68| -512,ssh-rsa,ssh
 0140 |2D-64-73-73-2C-78-35-30 39-76-33-2D-73-69-67-6E| -dss,x509v3-sign
 0150 |2D-72-73-61-2D-73-68-61 32-35-36-40-73-73-68-2E| -rsa-sha256@ssh.
 0160 |63-6F-6D-2C-78-35-30-39 76-33-2D-73-69-67-6E-2D| com,x509v3-sign-
 0170 |72-73-61-2C-78-35-30-39 76-33-2D-73-69-67-6E-2D| rsa,x509v3-sign-
 0180 |64-73-73-2C-65-63-64-73 61-2D-73-68-61-32-2D-6E| dss,ecdsa-sha2-n
 0190 |69-73-74-70-32-35-36-00 00-00-B9-61-65-73-32-35| istp256....aes25
 01A0 |36-2D-63-74-72-2C-61-65 73-31-39-32-2D-63-74-72| 6-ctr,aes192-ctr
 01B0 |2C-61-65-73-31-32-38-2D 63-74-72-2C-33-64-65-73| ,aes128-ctr,3des
 01C0 |2D-63-74-72-2C-74-77-6F 66-69-73-68-32-35-36-2D| -ctr,twofish256-
 01D0 |63-74-72-2C-74-77-6F-66 69-73-68-31-39-32-2D-63| ctr,twofish192-c
 01E0 |74-72-2C-74-77-6F-66-69 73-68-31-32-38-2D-63-74| tr,twofish128-ct
 01F0 |72-2C-61-65-73-32-35-36 2D-63-62-63-2C-61-65-73| r,aes256-cbc,aes
 0200 |31-39-32-2D-63-62-63-2C 61-65-73-31-32-38-2D-63| 192-cbc,aes128-c
 0210 |62-63-2C-33-64-65-73-2D 63-62-63-2C-74-77-6F-66| bc,3des-cbc,twof
 0220 |69-73-68-32-35-36-2D-63 62-63-2C-74-77-6F-66-69| ish256-cbc,twofi
 0230 |73-68-31-39-32-2D-63-62 63-2C-74-77-6F-66-69-73| sh192-cbc,twofis
 0240 |68-31-32-38-2D-63-62-63 2C-74-77-6F-66-69-73-68| h128-cbc,twofish
 0250 |2D-63-62-63-00-00-00-B9 61-65-73-32-35-36-2D-63| -cbc....aes256-c
 0260 |74-72-2C-61-65-73-31-39 32-2D-63-74-72-2C-61-65| tr,aes192-ctr,ae
 0270 |73-31-32-38-2D-63-74-72 2C-33-64-65-73-2D-63-74| s128-ctr,3des-ct
 0280 |72-2C-74-77-6F-66-69-73 68-32-35-36-2D-63-74-72| r,twofish256-ctr
 0290 |2C-74-77-6F-66-69-73-68 31-39-32-2D-63-74-72-2C| ,twofish192-ctr,
 02A0 |74-77-6F-66-69-73-68-31 32-38-2D-63-74-72-2C-61| twofish128-ctr,a
 02B0 |65-73-32-35-36-2D-63-62 63-2C-61-65-73-31-39-32| es256-cbc,aes192
 02C0 |2D-63-62-63-2C-61-65-73 31-32-38-2D-63-62-63-2C| -cbc,aes128-cbc,
 02D0 |33-64-65-73-2D-63-62-63 2C-74-77-6F-66-69-73-68| 3des-cbc,twofish
 02E0 |32-35-36-2D-63-62-63-2C 74-77-6F-66-69-73-68-31| 256-cbc,twofish1
 02F0 |39-32-2D-63-62-63-2C-74 77-6F-66-69-73-68-31-32| 92-cbc,twofish12
 0300 |38-2D-63-62-63-2C-74-77 6F-66-69-73-68-2D-63-62| 8-cbc,twofish-cb
 0310 |63-00-00-00-25-68-6D-61 63-2D-73-68-61-32-2D-32| c...%hmac-sha2-2
 0320 |35-36-2C-68-6D-61-63-2D 73-68-61-32-2D-35-31-32| 56,hmac-sha2-512
 0330 |2C-68-6D-61-63-2D-73-68 61-31-00-00-00-25-68-6D| ,hmac-sha1...%hm
 0340 |61-63-2D-73-68-61-32-2D 32-35-36-2C-68-6D-61-63| ac-sha2-256,hmac
 0350 |2D-73-68-61-32-2D-35-31 32-2C-68-6D-61-63-2D-73| -sha2-512,hmac-s
 0360 |68-61-31-00-00-00-1A-6E 6F-6E-65-2C-7A-6C-69-62| ha1....none,zlib
 0370 |2C-7A-6C-69-62-40-6F-70 65-6E-73-73-68-2E-63-6F| ,zlib@openssh.co
 0380 |6D-00-00-00-1A-6E-6F-6E 65-2C-7A-6C-69-62-2C-7A| m....none,zlib,z
 0390 |6C-69-62-40-6F-70-65-6E 73-73-68-2E-63-6F-6D-00| lib@openssh.com.
 03A0 |00-00-00-00-00-00-00-00 00-00-00-00            | ............
2021-05-19 12:54:28.374 DEBUG Sftp(1)[1] SSH: Group exchange.
2021-05-19 12:54:28.374 VERBOSE Sftp(1)[1] SSH: Sending packet SSH_MSG_KEX_DH_GEX_REQUEST (13 bytes).
 0000 |22-00-00-04-00-00-00-08 00-00-00-10-00         | "............
2021-05-19 12:54:28.470 VERBOSE Sftp(1)[6] SSH: Received packet SSH_MSG_KEX_31 (267 bytes).
 0000 |1F-00-00-01-01-00-EE-FD 0D-F9-B6-0C-BF-61-DD-BA| .............a..
 0010 |33-64-F8-B6-25-42-56-FC 2D-F3-21-1B-1D-80-F4-55| 3d..%BV.-.!....U
 0020 |68-16-C5-D6-29-B0-A4-02 28-86-1B-EE-7B-44-97-44| h...)...(...{D.D
 0030 |4D-D9-37-5E-F7-E8-99-7B B7-29-4C-6A-A0-97-A1-E5| M.7^...{.)Lj....
 0040 |AD-47-DB-72-C1-49-56-89 8F-D1-1C-A2-92-ED-76-21| .G.r.IV.......v!
 0050 |5E-09-D8-06-78-35-15-09 28-D5-F0-92-67-B2-92-14| ^...x5..(...g...
 0060 |15-9C-0C-B6-95-EE-CC-90 E0-5C-0E-21-37-4E-D6-F0| .........\.!7N..
 0070 |47-59-B3-F0-E1-62-EE-2C FB-C8-25-9B-C1-A1-F2-8C| GY...b.,..%.....
 0080 |80-97-00-E1-F3-EC-BA-24 A9-9A-78-BC-CC-6E-77-63| .......$..x..nwc
 0090 |48-70-58-37-F8-6A-2B-4A 34-DB-59-27-53-46-89-05| HpX7.j+J4.Y'SF..
 00A0 |17-98-02-78-8F-52-C3-37 E0-18-7F-69-C3-0D-52-C9| ...x.R.7...i..R.
 00B0 |95-46-29-0D-09-15-4E-AF EE-72-F9-CA-32-A9-89-AA| .F)...N..r..2...
 00C0 |45-AD-68-22-05-39-A6-AA 30-2A-EA-4D-CC-CE-7E-6F| E.h".9..0*.M..~o
 00D0 |CF-6D-EE-57-44-0A-45-11 B4-BA-F5-B4-83-2D-27-24| .m.WD.E......-'$
 00E0 |7F-DE-D2-31-16-84-13-B1 2A-0E-11-47-A5-70-6C-56| ...1....*..G.plV
 00F0 |23-87-88-A7-70-74-63-45 2E-12-54-F2-50-3B-A1-97| #...ptcE..T.P;..
 0100 |75-6F-22-4A-70-F3-00-00 00-01-02               | uo"Jp......
2021-05-19 12:54:28.475 DEBUG Sftp(1)[1] SSH: Negotiating key.
by (130 points)
2021-05-19 12:54:28.526 DEBUG Sftp(1)[1] SSH: Received 2048-bit Diffie-Hellman prime (minimum allowed size is 1024 bits).
2021-05-19 12:54:28.527 VERBOSE Sftp(1)[1] SSH: Sending packet SSH_MSG_KEX_DH_GEX_INIT (262 bytes).
 0000 |20-00-00-01-01-00-AA-41 7A-BC-D7-6C-42-9B-51-99|  ......Az..lB.Q.
 0010 |7D-15-6E-3B-48-A7-8B-A7 11-7D-3B-7B-22-22-D3-DF| }.n;H....};{""..
 0020 |F8-9F-87-FA-10-64-FE-2B 12-13-CC-F9-7B-9A-68-E5| .....d.+....{.h.
 0030 |5E-28-35-76-A6-73-A0-AA A4-01-2C-41-8B-21-C5-44| ^(5v.s....,A.!.D
 0040 |4F-03-94-6C-91-16-61-4D D1-33-AC-C0-60-28-82-1B| O..l..aM.3..`(..
 0050 |32-C4-BF-C3-BB-13-EF-23 4B-3B-F5-62-D1-AA-A4-89| 2......#K;.b....
 0060 |8A-F3-47-04-1E-F5-A0-1C E6-13-A9-E0-83-93-1D-B2| ..G.............
 0070 |69-4D-C7-3E-33-A9-C9-24 1A-81-36-5B-1A-1A-05-3D| iM.>3..$..6[...=
 0080 |B0-76-CA-FC-28-09-84-C5 7B-D7-76-50-20-05-3D-CE| .v..(...{.vP .=.
 0090 |72-7D-A6-A4-68-08-6C-23 30-98-9B-AD-61-E8-10-13| r}..h.l#0...a...
 00A0 |D8-AC-25-27-B9-FB-98-F1 51-22-74-2C-ED-5B-D6-A2| ..%'....Q"t,.[..
 00B0 |C5-DE-A3-72-45-6A-81-EA 94-8D-28-74-0E-07-F1-AC| ...rEj....(t....
 00C0 |1E-7C-91-B2-70-85-34-D2 DF-83-84-29-78-73-B0-C4| .|..p.4....)xs..
 00D0 |9F-F2-5A-0D-AC-0D-1C-A2 69-0F-7E-FB-7B-89-BC-85| ..Z.....i.~.{...
 00E0 |55-5A-15-AD-71-57-1B-8B 27-AC-2A-3F-A1-DA-48-B3| UZ..qW..'.*?..H.
 00F0 |A0-9F-F6-38-E3-59-56-F4 DF-90-B2-C2-77-12-05-7C| ...8.YV.....w..|
 0100 |3B-5D-2F-CC-D7-A1                              | ;]/...
2021-05-19 12:54:28.624 VERBOSE Sftp(1)[6] SSH: Received packet SSH_MSG_KEX_DH_GEX_REPLY (825 bytes).
 0000 |21-00-00-01-17-00-00-00 07-73-73-68-2D-72-73-61| !........ssh-rsa
 0010 |00-00-00-03-01-00-01-00 00-01-01-00-C2-DE-46-B7| ..............F.
 0020 |DE-27-B4-86-49-77-34-D4 A4-C4-42-E0-83-BB-51-34| .'..Iw4...B...Q4
 0030 |92-AA-DC-B1-F5-98-6C-2A 9A-9C-55-CD-70-27-E1-32| ......l*..U.p'.2
 0040 |C5-5C-DF-6B-77-A0-72-75 FE-58-8D-4D-08-13-06-DF| .\.kw.ru.X.M....
 0050 |64-FB-B3-EC-C2-74-33-F4 5A-BD-35-57-44-EA-43-4B| d....t3.Z.5WD.CK
 0060 |73-24-D1-25-18-DC-F1-7F 0D-B6-DF-62-7B-FC-95-9E| s$.%.......b{...
 0070 |AD-43-37-EA-F2-C3-AA-9B EF-97-9C-59-06-4D-13-21| .C7........Y.M.!
 0080 |EC-6E-96-B7-C7-D2-24-61 1A-7E-A0-4A-97-E5-CA-17| .n....$a.~.J....
 0090 |2C-05-2C-5D-90-7B-27-25 A8-2A-2E-42-8B-5B-28-9A| ,.,].{'%.*.B.[(.
 00A0 |E0-35-B7-01-FB-18-46-4E 9D-D9-3B-CB-F7-12-19-4B| .5....FN..;....K
 00B0 |C6-3B-B4-44-54-17-11-7A 43-AD-5C-A7-99-7F-3E-82| .;.DT..zC.\...>.
 00C0 |66-88-1B-65-0B-8A-C4-E3 F9-9F-87-2F-12-0C-9B-D5| f..e......./....
 00D0 |44-AA-A2-B2-4A-E1-CB-46 0A-78-58-65-F8-4A-C5-3E| D...J..F.xXe.J.>
 00E0 |84-BC-CE-4F-BF-1F-C5-A0 F3-56-C7-8E-D6-5A-1F-81| ...O.....V...Z..
 00F0 |A1-C9-2A-6F-45-99-FF-68 F2-79-1F-64-91-7E-5E-B4| ..*oE..h.y.d.~^.
 0100 |2E-B9-43-91-7C-C9-85-3C 53-7F-AF-36-D6-E4-F9-E9| ..C.|..<S..6....
 0110 |E9-9F-8F-46-56-F3-07-11 9D-7B-77-45-00-00-01-01| ...FV....{wE....
 0120 |00-96-75-C1-C2-4C-C3-CC F9-8E-B0-6A-1E-67-54-85| ..u..L.....j.gT.
 0130 |B4-A5-78-A8-52-F6-5B-4B F8-5C-D9-20-EB-10-89-C9| ..x.R.[K.\. ....
 0140 |F1-45-B6-8D-08-C6-6C-11 A9-DF-88-B2-DC-A5-2A-FA| .E....l.......*.
 0150 |FF-B7-F5-4A-6F-01-FD-B7 16-B9-AF-23-CC-0F-89-C4| ...Jo......#....
 0160 |0E-E4-A4-0D-92-D6-FE-AE 8C-E6-B9-19-30-00-B7-BE| ............0...
 0170 |0C-03-21-68-13-DC-6A-7C C3-27-8E-18-B6-77-AF-B5| ..!h..j|.'...w..
 0180 |92-AF-77-43-EA-30-7E-C4 4E-B9-62-B4-57-31-6A-CF| ..wC.0~.N.b.W1j.
 0190 |86-E0-ED-E9-A6-CD-DC-EE C9-9B-66-80-13-D3-42-DE| ..........f...B.
 01A0 |65-AE-97-89-1C-C9-02-AA 7D-A2-9D-3F-DF-1B-CF-2F| e.......}..?.../
 01B0 |25-62-29-F7-59-4D-E2-48 1B-BC-E5-D9-AF-0B-17-A8| %b).YM.H........
 01C0 |87-70-F9-A6-7D-CF-E1-86 E4-74-BB-F0-D6-C9-48-3F| .p..}....t....H?
 01D0 |E7-88-BC-53-58-07-F1-5D 41-BE-9B-00-EE-67-0F-68| ...SX..]A....g.h
 01E0 |2F-B8-2E-78-A7-58-52-B1 BD-F3-F4-1D-77-61-74-5B| /..x.XR.....wat[
 01F0 |2F-60-A6-80-86-22-85-5E 21-7D-80-42-C0-1E-52-26| /`...".^!}.B..R&
 0200 |C4-CA-51-45-16-CA-59-BC C0-94-C2-B3-78-45-AA-98| ..QE..Y.....xE..
 0210 |0E-9E-98-44-FE-42-03-4F 59-AC-2B-D6-E0-89-E3-9F| ...D.B.OY.+.....
 0220 |EC-00-00-01-14-00-00-00 0C-72-73-61-2D-73-68-61| .........rsa-sha
 0230 |32-2D-32-35-36-00-00-01 00-13-0B-AF-77-E0-A3-19| 2-256.......w...
 0240 |F9-83-4F-C3-48-C9-41-A5 E6-B6-79-17-DA-66-3D-B9| ..O.H.A...y..f=.
 0250 |54-35-59-D3-14-E4-8F-40 18-1B-94-A1-44-C4-42-A8| T5Y....@....D.B.
 0260 |C4-FF-55-59-08-14-51-D5 4C-6D-4D-44-B6-6C-C1-F1| ..UY..Q.LmMD.l..
 0270 |DE-F5-28-C6-3F-A5-CB-CD E0-6C-5E-30-F5-07-43-5D| ..(.?....l^0..C]
 0280 |B3-3B-9A-0F-76-A1-53-1D 34-63-C3-8C-5A-C5-42-5A| .;..v.S.4c..Z.BZ
 0290 |2E-11-5F-83-F0-46-4B-C5 E1-98-5C-9A-55-2C-30-80| .._..FK...\.U,0.
 02A0 |73-E3-09-BE-62-93-52-CE 1B-38-3D-CB-06-24-E9-AE| s...b.R..8=..$..
 02B0 |0B-15-4F-71-84-C3-78-99 A5-67-F6-89-33-C0-62-D0| ..Oq..x..g..3.b.
 02C0 |96-19-D3-E2-09-89-F1-AA 7C-18-E1-DA-53-52-10-B3| ........|...SR..
 02D0 |97-51-3A-0B-BA-1E-25-56 81-96-D5-BE-64-67-22-49| .Q:...%V....dg"I
 02E0 |8E-BC-5B-B2-A9-5A-D4-3A 9B-5B-81-4B-84-EF-4C-34| ..[..Z.:.[.K..L4
 02F0 |53-88-AB-3B-11-75-99-2C 47-A3-84-44-F7-87-66-DD| S..;.u.,G..D..f.
 0300 |51-2F-3B-65-77-80-CD-F1 44-EA-FB-4D-6F-3E-BB-38| Q/;ew...D..Mo>.8
 0310 |F9-DB-04-10-3D-4D-0D-D5 59-90-66-96-74-A4-44-6A| ....=M..Y.f.t.Dj
 0320 |20-E4-10-29-A0-C9-03-D2 A2-91-75-6F-FF-A9-08-8E|  ..)......uo....
 0330 |F4-FC-DB-26-EE-0E-6A-09 C8                     | ...&..j..
2021-05-19 12:54:28.624 VERBOSE Sftp(1)[6] SSH: Received packet SSH_MSG_NEWKEYS (1 bytes).
 0000 |15                                             | .
2021-05-19 12:54:28.736 DEBUG Sftp(1)[1] SSH: Validating signature.
2021-05-19 12:54:28.789 DEBUG Sftp(1)[1] SSH: Received 2048-bit RSA server key (minimum allowed size is 1024 bits).
2021-05-19 12:54:28.792 VERBOSE Sftp(1)[1] SSH: Sending packet SSH_MSG_NEWKEYS (1 bytes).
 0000 |15                                             | .
2021-05-19 12:54:28.813 INFO Sftp(1)[1] SSH: Negotiation finished.
2021-05-19 12:54:28.814 INFO Sftp(1)[1] Info: Server: SSH-2.0-OpenSSH_7.6
2021-05-19 12:54:28.814 INFO Sftp(1)[1] Info: Fingerprint (MD5): 56:3a:82:44:dc:5b:6f:59:f7:8c:6d:6b:3b:5a:d3:4a
2021-05-19 12:54:28.815 INFO Sftp(1)[1] Info: Fingerprint (SHA-256): yLHo/ASw4EjmygiF7gLa7dcKAlgf3xB5eaBslcPwAhg
2021-05-19 12:54:28.816 INFO Sftp(1)[1] Info: Cipher info: SSH 2.0, diffie-hellman-group-exchange-sha256, rsa-sha2-256, aes256-ctr/aes256-ctr, hmac-sha2-256/hmac-sha2-256
by (148k points)
Yeat one more request... Could you please post the log from the new version with only DiffieHellmanGroupExchangeSHA256 enabled?

_client.Settings.SshParameters.KeyExchangeAlgorithms = SshKeyExchangeAlgorithm.DiffieHellmanGroupExchangeSHA256;

Alternatively, would it be possible to temporarily enable access to the server's SSH port from our IP address? That way, we could quickly check this out ourselves with various versions and find out what's actually going on. We don't even need an account at the server, just an ability to connect and try to negotiate an SSH session.
by (130 points)
with the line:
_client.Settings.SshParameters.KeyExchangeAlgorithms = SshKeyExchangeAlgorithm.DiffieHellmanGroupExchangeSHA256;

here is the log:

2021-05-19 13:19:43.211 Opening log file.
2021-05-19 13:19:43.217 INFO FileLogWriter(1)[1] Info: Assembly: Rebex.Common R5.2 for .NET 4.6-4.8
2021-05-19 13:19:43.221 INFO FileLogWriter(1)[1] Info: Platform: Windows 6.2.9200 32-bit; CLR: 4.0.30319.42000
2021-05-19 13:19:43.224 DEBUG FileLogWriter(1)[1] Info: Culture: en; Windows-1252
2021-05-19 13:19:44.961 INFO Sftp(1)[1] Info: Connecting to 207.45.41.162:22 using Sftp.
2021-05-19 13:19:44.962 INFO Sftp(1)[1] Info: Assembly: Rebex.Sftp R5.2 for .NET 4.6-4.8
2021-05-19 13:19:44.963 INFO Sftp(1)[1] Info: Platform: Windows 6.2.9200 32-bit; CLR: 4.0.30319.42000
2021-05-19 13:19:44.963 DEBUG Sftp(1)[1] Info: Culture: en; Windows-1252
2021-05-19 13:19:44.982 DEBUG Sftp(1)[1] Proxy: Connecting to 207.45.41.162:22 (no proxy).
2021-05-19 13:19:45.057 DEBUG Sftp(1)[1] Proxy: Connection established.
2021-05-19 13:19:45.068 VERBOSE Sftp(1)[1] SSH: Sending data:
 0000 |53-53-48-2D-32-2E-30-2D 52-65-62-65-78-53-53-48| SSH-2.0-RebexSSH
 0010 |5F-35-2E-30-2E-37-38-30 30-2E-30-0D-0A         | _5.0.7800.0..
2021-05-19 13:19:45.269 VERBOSE Sftp(1)[1] SSH: Received data:
 0000 |53-53-48-2D-32-2E-30-2D 4F-70-65-6E-53-53-48-5F| SSH-2.0-OpenSSH_
 0010 |37-2E-36-0D-0A                                 | 7.6..
2021-05-19 13:19:45.269 DEBUG Sftp(1)[1] SSH: Server is 'SSH-2.0-OpenSSH_7.6'.
2021-05-19 13:19:45.278 INFO Sftp(1)[1] SSH: Negotiation started.
2021-05-19 13:19:45.311 VERBOSE Sftp(1)[7] SSH: Received packet SSH_MSG_KEXINIT (975 bytes).
 0000 |14-A3-AB-89-2C-91-CA-2A DC-72-D8-12-1E-9D-D6-B4| ....,..*.r......
 0010 |C3-00-00-01-40-63-75-72 76-65-32-35-35-31-39-2D| ....@curve25519-
 0020 |73-68-61-32-35-36-2C-63 75-72-76-65-32-35-35-31| sha256,curve2551
 0030 |39-2D-73-68-61-32-35-36 40-6C-69-62-73-73-68-2E| 9-sha256@libssh.
 0040 |6F-72-67-2C-65-63-64-68 2D-73-68-61-32-2D-6E-69| org,ecdh-sha2-ni
 0050 |73-74-70-32-35-36-2C-65 63-64-68-2D-73-68-61-32| stp256,ecdh-sha2
 0060 |2D-6E-69-73-74-70-33-38 34-2C-65-63-64-68-2D-73| -nistp384,ecdh-s
 0070 |68-61-32-2D-6E-69-73-74 70-35-32-31-2C-64-69-66| ha2-nistp521,dif
 0080 |66-69-65-2D-68-65-6C-6C 6D-61-6E-2D-67-72-6F-75| fie-hellman-grou
 0090 |70-2D-65-78-63-68-61-6E 67-65-2D-73-68-61-32-35| p-exchange-sha25
 00A0 |36-2C-64-69-66-66-69-65 2D-68-65-6C-6C-6D-61-6E| 6,diffie-hellman
 00B0 |2D-67-72-6F-75-70-31-36 2D-73-68-61-35-31-32-2C| -group16-sha512,
 00C0 |64-69-66-66-69-65-2D-68 65-6C-6C-6D-61-6E-2D-67| diffie-hellman-g
 00D0 |72-6F-75-70-31-38-2D-73 68-61-35-31-32-2C-64-69| roup18-sha512,di
 00E0 |66-66-69-65-2D-68-65-6C 6C-6D-61-6E-2D-67-72-6F| ffie-hellman-gro
 00F0 |75-70-2D-65-78-63-68-61 6E-67-65-2D-73-68-61-31| up-exchange-sha1
 0100 |2C-64-69-66-66-69-65-2D 68-65-6C-6C-6D-61-6E-2D| ,diffie-hellman-
 0110 |67-72-6F-75-70-31-34-2D 73-68-61-32-35-36-2C-64| group14-sha256,d
 0120 |69-66-66-69-65-2D-68-65 6C-6C-6D-61-6E-2D-67-72| iffie-hellman-gr
 0130 |6F-75-70-31-34-2D-73-68 61-31-2C-64-69-66-66-69| oup14-sha1,diffi
 0140 |65-2D-68-65-6C-6C-6D-61 6E-2D-67-72-6F-75-70-31| e-hellman-group1
 0150 |2D-73-68-61-31-00-00-00 21-73-73-68-2D-72-73-61| -sha1...!ssh-rsa
 0160 |2C-72-73-61-2D-73-68-61 32-2D-35-31-32-2C-72-73| ,rsa-sha2-512,rs
 0170 |61-2D-73-68-61-32-2D-32 35-36-00-00-00-6F-61-65| a-sha2-256...oae
 0180 |73-31-32-38-2D-63-74-72 2C-61-65-73-31-39-32-2D| s128-ctr,aes192-
 0190 |63-74-72-2C-61-65-73-32 35-36-2D-63-74-72-2C-61| ctr,aes256-ctr,a
 01A0 |65-73-31-32-38-2D-63-62 63-2C-61-65-73-31-39-32| es128-cbc,aes192
 01B0 |2D-63-62-63-2C-61-65-73 32-35-36-2D-63-62-63-2C| -cbc,aes256-cbc,
 01C0 |61-65-73-32-35-36-2D-67 63-6D-40-6F-70-65-6E-73| aes256-gcm@opens
 01D0 |73-68-2E-63-6F-6D-2C-61 65-73-31-32-38-2D-67-63| sh.com,aes128-gc
 01E0 |6D-40-6F-70-65-6E-73-73 68-2E-63-6F-6D-00-00-00| m@openssh.com...
 01F0 |6F-61-65-73-31-32-38-2D 63-74-72-2C-61-65-73-31| oaes128-ctr,aes1
 0200 |39-32-2D-63-74-72-2C-61 65-73-32-35-36-2D-63-74| 92-ctr,aes256-ct
 0210 |72-2C-61-65-73-31-32-38 2D-63-62-63-2C-61-65-73| r,aes128-cbc,aes
 0220 |31-39-32-2D-63-62-63-2C 61-65-73-32-35-36-2D-63| 192-cbc,aes256-c
 0230 |62-63-2C-61-65-73-32-35 36-2D-67-63-6D-40-6F-70| bc,aes256-gcm@op
 0240 |65-6E-73-73-68-2E-63-6F 6D-2C-61-65-73-31-32-38| enssh.com,aes128
 0250 |2D-67-63-6D-40-6F-70-65 6E-73-73-68-2E-63-6F-6D| -gcm@openssh.com
 0260 |00-00-00-A5-68-6D-61-63 2D-73-68-61-31-2D-65-74| ....hmac-sha1-et
 0270 |6D-40-6F-70-65-6E-73-73 68-2E-63-6F-6D-2C-68-6D| m@openssh.com,hm
 0280 |61-63-2D-73-68-61-32-2D 32-35-36-2D-65-74-6D-40| ac-sha2-256-etm@
 0290 |6F-70-65-6E-73-73-68-2E 63-6F-6D-2C-68-6D-61-63| openssh.com,hmac
 02A0 |2D-73-68-61-32-2D-35-31 32-2D-65-74-6D-40-6F-70| -sha2-512-etm@op
 02B0 |65-6E-73-73-68-2E-63-6F 6D-2C-68-6D-61-63-2D-73| enssh.com,hmac-s
 02C0 |68-61-31-2D-39-36-2D-65 74-6D-40-6F-70-65-6E-73| ha1-96-etm@opens
 02D0 |73-68-2E-63-6F-6D-2C-68 6D-61-63-2D-73-68-61-31| sh.com,hmac-sha1
 02E0 |2C-68-6D-61-63-2D-73-68 61-32-2D-32-35-36-2C-68| ,hmac-sha2-256,h
 02F0 |6D-61-63-2D-73-68-61-32 2D-35-31-32-2C-68-6D-61| mac-sha2-512,hma
 0300 |63-2D-73-68-61-31-2D-39 36-00-00-00-A5-68-6D-61| c-sha1-96....hma
 0310 |63-2D-73-68-61-31-2D-65 74-6D-40-6F-70-65-6E-73| c-sha1-etm@opens
 0320 |73-68-2E-63-6F-6D-2C-68 6D-61-63-2D-73-68-61-32| sh.com,hmac-sha2
 0330 |2D-32-35-36-2D-65-74-6D 40-6F-70-65-6E-73-73-68| -256-etm@openssh
 0340 |2E-63-6F-6D-2C-68-6D-61 63-2D-73-68-61-32-2D-35| .com,hmac-sha2-5
 0350 |31-32-2D-65-74-6D-40-6F 70-65-6E-73-73-68-2E-63| 12-etm@openssh.c
 0360 |6F-6D-2C-68-6D-61-63-2D 73-68-61-31-2D-39-36-2D| om,hmac-sha1-96-
 0370 |65-74-6D-40-6F-70-65-6E 73-73-68-2E-63-6F-6D-2C| etm@openssh.com,
 0380 |68-6D-61-63-2D-73-68-61 31-2C-68-6D-61-63-2D-73| hmac-sha1,hmac-s
 0390 |68-61-32-2D-32-35-36-2C 68-6D-61-63-2D-73-68-61| ha2-256,hmac-sha
 03A0 |32-2D-35-31-32-2C-68-6D 61-63-2D-73-68-61-31-2D| 2-512,hmac-sha1-
 03B0 |39-36-00-00-00-04-6E-6F 6E-65-00-00-00-04-6E-6F| 96....none....no
 03C0 |6E-65-00-00-00-00-00-00 00-00-00-00-00-00-00   | ne.............
by (130 points)
2021-05-19 13:19:45.406 VERBOSE Sftp(1)[1] SSH: Sending packet SSH_MSG_KEXINIT (1036 bytes).
 0000 |14-C5-3B-93-1F-A7-E7-71 23-6E-A9-CD-ED-2C-40-08| ..;....q#n...,@.
 0010 |5D-00-00-00-2F-64-69-66 66-69-65-2D-68-65-6C-6C| ].../diffie-hell
 0020 |6D-61-6E-2D-67-72-6F-75 70-2D-65-78-63-68-61-6E| man-group-exchan
 0030 |67-65-2D-73-68-61-32-35 36-2C-65-78-74-2D-69-6E| ge-sha256,ext-in
 0040 |66-6F-2D-63-00-00-00-9F 72-73-61-2D-73-68-61-32| fo-c....rsa-sha2
 0050 |2D-32-35-36-2C-73-73-68 2D-72-73-61-2D-73-68-61| -256,ssh-rsa-sha
 0060 |32-35-36-40-73-73-68-2E 63-6F-6D-2C-72-73-61-2D| 256@ssh.com,rsa-
 0070 |73-68-61-32-2D-35-31-32 2C-73-73-68-2D-72-73-61| sha2-512,ssh-rsa
 0080 |2C-73-73-68-2D-64-73-73 2C-78-35-30-39-76-33-2D| ,ssh-dss,x509v3-
 0090 |73-69-67-6E-2D-72-73-61 2D-73-68-61-32-35-36-40| sign-rsa-sha256@
 00A0 |73-73-68-2E-63-6F-6D-2C 78-35-30-39-76-33-2D-73| ssh.com,x509v3-s
 00B0 |69-67-6E-2D-72-73-61-2C 78-35-30-39-76-33-2D-73| ign-rsa,x509v3-s
 00C0 |69-67-6E-2D-64-73-73-2C 73-73-68-2D-65-64-32-35| ign-dss,ssh-ed25
 00D0 |35-31-39-2C-65-63-64-73 61-2D-73-68-61-32-2D-6E| 519,ecdsa-sha2-n
 00E0 |69-73-74-70-32-35-36-00 00-01-05-61-65-73-32-35| istp256....aes25
 00F0 |36-2D-67-63-6D-40-6F-70 65-6E-73-73-68-2E-63-6F| 6-gcm@openssh.co
 0100 |6D-2C-61-65-73-31-32-38 2D-67-63-6D-40-6F-70-65| m,aes128-gcm@ope
 0110 |6E-73-73-68-2E-63-6F-6D 2C-61-65-73-32-35-36-2D| nssh.com,aes256-
 0120 |63-74-72-2C-61-65-73-31 39-32-2D-63-74-72-2C-61| ctr,aes192-ctr,a
 0130 |65-73-31-32-38-2D-63-74 72-2C-33-64-65-73-2D-63| es128-ctr,3des-c
 0140 |74-72-2C-74-77-6F-66-69 73-68-32-35-36-2D-63-74| tr,twofish256-ct
 0150 |72-2C-74-77-6F-66-69-73 68-31-39-32-2D-63-74-72| r,twofish192-ctr
 0160 |2C-74-77-6F-66-69-73-68 31-32-38-2D-63-74-72-2C| ,twofish128-ctr,
 0170 |61-65-73-32-35-36-2D-63 62-63-2C-61-65-73-31-39| aes256-cbc,aes19
 0180 |32-2D-63-62-63-2C-61-65 73-31-32-38-2D-63-62-63| 2-cbc,aes128-cbc
 0190 |2C-33-64-65-73-2D-63-62 63-2C-74-77-6F-66-69-73| ,3des-cbc,twofis
 01A0 |68-32-35-36-2D-63-62-63 2C-74-77-6F-66-69-73-68| h256-cbc,twofish
 01B0 |31-39-32-2D-63-62-63-2C 74-77-6F-66-69-73-68-31| 192-cbc,twofish1
 01C0 |32-38-2D-63-62-63-2C-74 77-6F-66-69-73-68-2D-63| 28-cbc,twofish-c
 01D0 |62-63-2C-63-68-61-63-68 61-32-30-2D-70-6F-6C-79| bc,chacha20-poly
 01E0 |31-33-30-35-40-6F-70-65 6E-73-73-68-2E-63-6F-6D| 1305@openssh.com
 01F0 |00-00-01-05-61-65-73-32 35-36-2D-67-63-6D-40-6F| ....aes256-gcm@o
 0200 |70-65-6E-73-73-68-2E-63 6F-6D-2C-61-65-73-31-32| penssh.com,aes12
 0210 |38-2D-67-63-6D-40-6F-70 65-6E-73-73-68-2E-63-6F| 8-gcm@openssh.co
 0220 |6D-2C-61-65-73-32-35-36 2D-63-74-72-2C-61-65-73| m,aes256-ctr,aes
 0230 |31-39-32-2D-63-74-72-2C 61-65-73-31-32-38-2D-63| 192-ctr,aes128-c
 0240 |74-72-2C-33-64-65-73-2D 63-74-72-2C-74-77-6F-66| tr,3des-ctr,twof
 0250 |69-73-68-32-35-36-2D-63 74-72-2C-74-77-6F-66-69| ish256-ctr,twofi
 0260 |73-68-31-39-32-2D-63-74 72-2C-74-77-6F-66-69-73| sh192-ctr,twofis
 0270 |68-31-32-38-2D-63-74-72 2C-61-65-73-32-35-36-2D| h128-ctr,aes256-
 0280 |63-62-63-2C-61-65-73-31 39-32-2D-63-62-63-2C-61| cbc,aes192-cbc,a
 0290 |65-73-31-32-38-2D-63-62 63-2C-33-64-65-73-2D-63| es128-cbc,3des-c
 02A0 |62-63-2C-74-77-6F-66-69 73-68-32-35-36-2D-63-62| bc,twofish256-cb
 02B0 |63-2C-74-77-6F-66-69-73 68-31-39-32-2D-63-62-63| c,twofish192-cbc
 02C0 |2C-74-77-6F-66-69-73-68 31-32-38-2D-63-62-63-2C| ,twofish128-cbc,
 02D0 |74-77-6F-66-69-73-68-2D 63-62-63-2C-63-68-61-63| twofish-cbc,chac
 02E0 |68-61-32-30-2D-70-6F-6C 79-31-33-30-35-40-6F-70| ha20-poly1305@op
 02F0 |65-6E-73-73-68-2E-63-6F 6D-00-00-00-61-68-6D-61| enssh.com...ahma
 0300 |63-2D-73-68-61-32-2D-32 35-36-2D-65-74-6D-40-6F| c-sha2-256-etm@o
 0310 |70-65-6E-73-73-68-2E-63 6F-6D-2C-68-6D-61-63-2D| penssh.com,hmac-
 0320 |73-68-61-32-2D-32-35-36 2C-68-6D-61-63-2D-73-68| sha2-256,hmac-sh
 0330 |61-32-2D-35-31-32-2D-65 74-6D-40-6F-70-65-6E-73| a2-512-etm@opens
 0340 |73-68-2E-63-6F-6D-2C-68 6D-61-63-2D-73-68-61-32| sh.com,hmac-sha2
 0350 |2D-35-31-32-2C-68-6D-61 63-2D-73-68-61-31-00-00| -512,hmac-sha1..
 0360 |00-61-68-6D-61-63-2D-73 68-61-32-2D-32-35-36-2D| .ahmac-sha2-256-
 0370 |65-74-6D-40-6F-70-65-6E 73-73-68-2E-63-6F-6D-2C| etm@openssh.com,
 0380 |68-6D-61-63-2D-73-68-61 32-2D-32-35-36-2C-68-6D| hmac-sha2-256,hm
 0390 |61-63-2D-73-68-61-32-2D 35-31-32-2D-65-74-6D-40| ac-sha2-512-etm@
 03A0 |6F-70-65-6E-73-73-68-2E 63-6F-6D-2C-68-6D-61-63| openssh.com,hmac
 03B0 |2D-73-68-61-32-2D-35-31 32-2C-68-6D-61-63-2D-73| -sha2-512,hmac-s
 03C0 |68-61-31-00-00-00-1A-6E 6F-6E-65-2C-7A-6C-69-62| ha1....none,zlib
 03D0 |2C-7A-6C-69-62-40-6F-70 65-6E-73-73-68-2E-63-6F| ,zlib@openssh.co
 03E0 |6D-00-00-00-1A-6E-6F-6E 65-2C-7A-6C-69-62-2C-7A| m....none,zlib,z
 03F0 |6C-69-62-40-6F-70-65-6E 73-73-68-2E-63-6F-6D-00| lib@openssh.com.
 0400 |00-00-00-00-00-00-00-00 00-00-00-00            | ............
2021-05-19 13:19:45.421 DEBUG Sftp(1)[1] SSH: Group exchange.
2021-05-19 13:19:45.422 VERBOSE Sftp(1)[1] SSH: Sending packet SSH_MSG_KEX_DH_GEX_REQUEST (13 bytes).
 0000 |22-00-00-04-00-00-00-08 00-00-00-10-00         | "............
2021-05-19 13:19:45.489 DEBUG Sftp(1)[7] SSH: SSH connection closed.
2021-05-19 13:19:45.496 ERROR Sftp(1)[1] SSH: Negotiation failed. The connection was closed by the server.
2021-05-19 13:19:45.514 ERROR Sftp(1)[1] Info: Rebex.Net.SshException: The connection was closed by the server.
   at Rebex.Net.SshSession.zdtde[T,S](nryaa`2 p0, Int32 p1, wvmhp p2, S p3, T p4, T p5)
   at Rebex.Net.SshSession.yntgu[T,S](nryaa`2 p0, S p1)
   at Rebex.Net.SshSession.fdetz(wjwzv p0)
   at fbmqn.mtqah.acyly(SshSession p0, Byte[] p1, Byte[] p2, Byte[] p3, Byte[] p4, vuqti& p5, Byte[]& p6, SshPublicKey& p7)
   at Rebex.Net.SshSession.dgduw(Byte[] p0)
   at Rebex.Net.SshSession.Negotiate()
   at Rebex.Net.Sftp.fbmqn.mjgdl.gmlbl(rqghl p0, Boolean p1)
   at Rebex.Net.Sftp.yhxos(String p0, Int32 p1, SshParameters p2, rqghl p3)
by (130 points)
I would love to give you access, but it's unlikely. I'll see if we can convince the end point.
by (148k points)
OK, I believe we'll eventually resolve this anyway, even though it might take a bit of additional communication.

Also, thanks for all the logs!

This is where the behavior starts to differ when using DiffieHellmanGroupExchangeSHA256 in both versions, despite the actualy contents of that packet being identical:

v5.0.7800.0:

2021-05-19 13:19:45.421 DEBUG Sftp(1)[1] SSH: Group exchange.
2021-05-19 13:19:45.422 VERBOSE Sftp(1)[1] SSH: Sending packet SSH_MSG_KEX_DH_GEX_REQUEST (13 bytes).
 0000 |22-00-00-04-00-00-00-08 00-00-00-10-00         | "............
2021-05-19 13:19:45.489 DEBUG Sftp(1)[7] SSH: SSH connection closed.

v3.0.6586.0:

2021-05-19 12:54:28.374 DEBUG Sftp(1)[1] SSH: Group exchange.
2021-05-19 12:54:28.374 VERBOSE Sftp(1)[1] SSH: Sending packet SSH_MSG_KEX_DH_GEX_REQUEST (13 bytes).
 0000 |22-00-00-04-00-00-00-08 00-00-00-10-00         | "............
2021-05-19 12:54:28.470 VERBOSE Sftp(1)[6] SSH: Received packet SSH_MSG_KEX_31 (267 bytes).
...
(success)

This is very strange, because it suggests there is not really anything wrong with the key exchange (which is just about to get started - client is just announcing its preference for the key size here).
Instead, my guess is that perhaps the server does not like something about the client's SSH_MSG_KEXINIT packet, and drops the connection.

Next, we'll try to configure v5.0.7800.0 to send a SSH_MSG_KEXINIT that is identical to that of v3.0.6586.0 and see whether that helps. If we change the initial software identification string as well,
that will make the new version indistinguishable from the old version to the server, so hopefully that would lead somewhere. We'll look into this tomorrow - sorry for the delay!
by (130 points)
edited by
Thanks for your time on this. I've discussed this with the relevant people and the best access we could give you is to do a live session where I share the screen of the machine that has connectivity and we interactively go from there. Also if there are other tools like network sniffers that would give you more relevant information we can go down that route.  if that would be useful let me know. If we can continue this with email that would be great as I would prefer to discuss this on something other then a public forum due to security concerns.

Thanks.
by (148k points)
OK, in that case, let's first try configuring v5.0.7800.0 to send SSH_MSG_KEXINIT identical to that of v3.0.6586.0 tomorrow (I'll send you an email with relevant code from support@rebex.net and notify you here as well), and if that doesn't get us closer to a solution, we'll try the live session later.
by (148k points)
I just sent you an e-mail with instructions. Please give it a try and let me know whether that works around the issue.
by (130 points)
Thanks the specific configuration you provided fixed the issue. I did not have to resort to using the hotfix.
Thanks for the really great and responsive support.
by (148k points)
Thanks for letting us know!
Still, it might be useful to know which particular setting triggers the issue. Would it be possible to try removing those 7 settings one-by-one to find out which ones are actually necessary to work around the problem? Let me know by e-mail if you prefer.
...