I added logging and test.
Synology FTPS server do not answer the command Abort duning 10 seconds. (AbortTimeout = 10000)
log:
2017-08-04 13:31:21.662 Opening log file.
2017-08-04 13:31:21.662 Using FileLogWriter version 2.5.6391.0.
2017-08-04 13:31:21.662 INFO RaiFtp(55)[19] Info: Connecting to raidrive.synology.me:921 using Ftp.
2017-08-04 13:31:21.662 INFO RaiFtp(55)[19] Info: Assembly: Rebex.Ftp 2017 R4 for .NET 4.0-4.7 (Trial)
2017-08-04 13:31:21.662 DEBUG RaiFtp(55)[19] Info: Platform: Windows 10.0.15063 64-bit; CLR: 4.0.30319.42000
2017-08-04 13:31:21.662 DEBUG RaiFtp(55)[19] Info: Culture: ko; ks_c_5601-1987
2017-08-04 13:31:21.662 INFO RaiFtp(55)[19] Info: Using proxy none.
2017-08-04 13:31:21.662 DEBUG ProxySocket(178)[19] Socket: Resolving 'raidrive.synology.me'.
2017-08-04 13:31:21.662 DEBUG ProxySocket(178)[12] Socket: Connecting to 210.106.56.71:921.
2017-08-04 13:31:21.672 DEBUG RaiFtp(55)[19] Info: Connection succeeded.
2017-08-04 13:31:21.687 INFO RaiFtp(55)[19] Response: 220 here FTP server ready.
2017-08-04 13:31:21.687 INFO RaiFtp(55)[19] Command: AUTH TLS
...
2017-08-04 13:31:21.731 INFO RaiFtp(55)[19] TLS: Connection secured using cipher: TLS 1.2, RSA, 256bit AES in CBC mode, SHA256
2017-08-04 13:31:21.731 DEBUG RaiFtp(55)[19] TLS: Session ID:
0000 |7D-0B-D8-35-B4-74-95-6F 48-FB-3B-1C-32-62-ED-77| }..5.t.oH.;.2b.w
0010 |88-2F-6F-C9-BA-2A-63-CC 4B-26-88-A7-18-80-6A-F3| ./o..*c.K&....j.
2017-08-04 13:31:21.731 DEBUG RaiFtp(55)[19] Info: Control connection upgraded to TLS/SSL.
2017-08-04 13:31:21.731 INFO RaiFtp(55)[19] Command: USER lanopk
...
2017-08-04 13:31:22.045 INFO RaiFtp(55)[19] Command: OPTS UTF8 ON
2017-08-04 13:31:22.053 INFO RaiFtp(55)[19] Response: 200 OK, UTF-8 enabled
2017-08-04 13:31:22.059 DEBUG RaiFtp(55)[19] Info: Starting data transfer.
2017-08-04 13:31:22.059 INFO RaiFtp(55)[19] Command: PBSZ 0
2017-08-04 13:31:22.078 INFO RaiFtp(55)[19] Response: 200 PBSZ command successful (PBSZ=0).
2017-08-04 13:31:22.078 INFO RaiFtp(55)[19] Command: PROT P
2017-08-04 13:31:22.088 INFO RaiFtp(55)[19] Response: 200 Protection level set to Private.
2017-08-04 13:31:22.088 INFO RaiFtp(55)[19] Command: TYPE I
2017-08-04 13:31:22.104 INFO RaiFtp(55)[19] Response: 200 Type set to I.
2017-08-04 13:31:22.104 INFO RaiFtp(55)[19] Command: PASV
2017-08-04 13:31:22.112 INFO RaiFtp(55)[19] Response: 227 Entering Passive Mode (192,168,200,40,216,242)
2017-08-04 13:31:22.112 INFO RaiFtp(55)[19] Info: Using server address 210.106.56.71 instead of received address 192.168.200.40.
2017-08-04 13:31:22.112 INFO RaiFtp(55)[19] Command: REST 253755392
2017-08-04 13:31:22.140 INFO RaiFtp(55)[19] Response: 350 Restarting at 253755392. Send STORE or RETRIEVE to initiate transfer.
2017-08-04 13:31:22.140 DEBUG RaiFtp(55)[19] Info: Establishing data connection to 210.106.56.71:55538.
2017-08-04 13:31:22.140 DEBUG ProxySocket(182)[19] Socket: Connecting to 210.106.56.71:55538.
2017-08-04 13:31:22.149 DEBUG RaiFtp(55)[19] Info: Established data connection from 192.168.0.19:6823.
2017-08-04 13:31:22.149 INFO RaiFtp(55)[19] Command: RETR /Test/ReadWrite/TestFile.mp4
2017-08-04 13:31:22.163 INFO RaiFtp(55)[19] Response: 150 Opening BINARY mode SSL data connection for '/Test/ReadWrite/TestFile.mp4' (256197311 bytes).
2017-08-04 13:31:22.163 DEBUG RaiFtp(55)[19] Info: Upgrading data connection to TLS/SSL.
2017-08-04 13:31:22.163 DEBUG RaiFtp(55)[19] TLS: Data connection: Asking the server to resume session ID:
0000 |7D-0B-D8-35-B4-74-95-6F 48-FB-3B-1C-32-62-ED-77| }..5.t.oH.;.2b.w
0010 |88-2F-6F-C9-BA-2A-63-CC 4B-26-88-A7-18-80-6A-F3| ./o..*c.K&....j.
2017-08-04 13:31:22.163 INFO RaiFtp(55)[19] TLS: Data connection: State StateChange:Negotiating
...
2017-08-04 13:31:22.182 DEBUG RaiFtp(55)[19] TLS: Data connection: Session ID:
0000 |7D-0B-D8-35-B4-74-95-6F 48-FB-3B-1C-32-62-ED-77| }..5.t.oH.;.2b.w
0010 |88-2F-6F-C9-BA-2A-63-CC 4B-26-88-A7-18-80-6A-F3| ./o..*c.K&....j.
2017-08-04 13:31:22.182 DEBUG RaiFtp(55)[19] Info: Data connection upgraded to TLS/SSL.
2017-08-04 13:31:24.974 INFO RaiFtp(55)[19] Info: Aborting operation.
2017-08-04 13:31:24.977 DEBUG RaiFtp(55)[19] Info: Aborting downloading data connection.
2017-08-04 13:31:24.977 INFO RaiFtp(55)[19] Info: Aborting download.
2017-08-04 13:31:24.977 INFO RaiFtp(55)[19] Command: ABOR
2017-08-04 13:31:35.981 INFO RaiFtp(55)[19] Info: Timing out because of an abort method.
2017-08-04 13:31:35.981 DEBUG RaiFtp(55)[19] Info: Unable to abort, closing data connection instead.
2017-08-04 13:31:35.981 DEBUG RaiFtp(55)[19] TLS: Data connection: Closing TLS socket.
2017-08-04 13:31:35.981 INFO RaiFtp(55)[19] TLS: Data connection: Alert Alert:Alert was sent.
2017-08-04 13:31:35.981 INFO RaiFtp(55)[19] TLS: Data connection: State StateChange:Closed
2017-08-04 13:31:35.992 DEBUG RaiFtp(55)[19] TLS: TLS socket error, 0 bytes of data were received.
2017-08-04 13:31:35.992 DEBUG RaiFtp(55)[19] TLS: TLS socket was closed: System.Net.Sockets.SocketException (0x80004005): 현재 연결은 원격 호스트에 의해 강제로 끊겼습니다 => means : current connection was closed forcely by remote host.
위치: System.Net.Sockets.Socket.Receive(Byte[] buffer, Int32 offset, Int32 size, SocketFlags socketFlags)
위치: Rebex.Net.BPW.RX(Int32 D)
위치: Rebex.Net.BPW.HX()
위치: Rebex.Net.BPW.UX()
2017-08-04 13:31:35.992 INFO RaiFtp(55)[19] TLS: Alert Alert:Alert was sent.
2017-08-04 13:31:35.992 INFO RaiFtp(55)[19] TLS: State StateChange:Closed
2017-08-04 13:31:35.992 DEBUG RaiFtp(55)[19] Info: Control connection was forcibly closed.
2017-08-04 13:31:35.992 DEBUG RaiFtp(55)[19] TLS: Closing TLS socket.
2017-08-04 13:31:35.993 ERROR RaiFtp(55)[19] Info: Error while closing data connection: Rebex.Net.FtpException: The server has closed the connection. ---> Rebex.Net.FtpException: The server has closed the connection. ---> Rebex.Net.TlsException: Connection was closed by the remote connection end. ---> System.Net.Sockets.SocketException: 현재 연결은 원격 호스트에 의해 강제로 끊겼습니다
위치: System.Net.Sockets.Socket.Receive(Byte[] buffer, Int32 offset, Int32 size, SocketFlags socketFlags)
위치: Rebex.Net.BPW.RX(Int32 D)
위치: Rebex.Net.BPW.HX()
위치: Rebex.Net.BPW.UX()
--- 내부 예외 스택 추적의 끝 ---
위치: Rebex.Net.BPW.UX()
위치: Rebex.Net.TlsSocket.Poll(Int32 microSeconds, SocketSelectMode mode)
위치: Rebex.Net.QXW.XT(Int32 D)
--- 내부 예외 스택 추적의 끝 ---
위치: Rebex.Net.QXW.XT(Int32 D)
위치: Rebex.Net.QXW.LT(String& D, Int32 N)
위치: Rebex.Net.QXW.AT(Int32 D)
위치: Rebex.Net.Ftp.DM(Int32 D, Boolean N)
위치: Rebex.Net.Ftp.RM(HOW D, Int64 N, Boolean T)
--- 내부 예외 스택 추적의 끝 ---
위치: Rebex.Net.Ftp.RM(HOW D, Int64 N, Boolean T)
위치: Rebex.Net.HOW.ZW()
2017-08-04 13:31:36.018 ERROR RaiFtp(55)[19] Info: System.InvalidOperationException: Not connected to the server.
위치: Rebex.Net.Ftp.TR()
위치: Rebex.Net.Ftp.BM(String D, SeekOrigin N, Int64 T)
2017-08-04 13:31:38.047 ERROR RaiFtp(55)[19] Info: System.InvalidOperationException: Not connected to the server.
위치: Rebex.Net.Ftp.TR()
위치: Rebex.Net.Ftp.BM(String D, SeekOrigin N, Int64 T)
2017-08-04 13:31:42.063 ERROR RaiFtp(55)[19] Info: System.InvalidOperationException: Not connected to the server.
위치: Rebex.Net.Ftp.TR()
위치: Rebex.Net.Ftp.BM(String D, SeekOrigin N, Int64 T)
2017-08-04 13:31:50.079 ERROR RaiFtp(55)[19] Info: System.InvalidOperationException: Not connected to the server.
위치: Rebex.Net.Ftp.TR()
위치: Rebex.Net.Ftp.BM(String D, SeekOrigin N, Int64 T)
2017-08-04 13:32:06.096 ERROR RaiFtp(55)[19] Info: System.InvalidOperationException: Not connected to the server.
위치: Rebex.Net.Ftp.TR()
위치: Rebex.Net.Ftp.BM(String D, SeekOrigin N, Int64 T)