Thank you for the log file. I have already replaied to it. I am copying my answer here, for other users.
I suggest you following steps:
1) Try to adjust size of data packets to smaller value by setting:
client.Settings.UploadBufferSize = 4 * 1024;
or to greater value:
client.Settings.UploadBufferSize = 48 * 1024;
2) Disable firewall and antivirus – sometimes this kind of software breaks existing connections.
3) Update Rebex assemblies to the latest version.
From the log file I can see that client version is Sftp 3.0.5171.0, which was released at 2014-02-26.
The current version is 3.0.6291.0, which was released at 2017-03-22.
• If you have active support contract, you can download current full version from https://www.rebex.net/protected/
• If you don’t have active support contract, you can download current trial version from https://www.rebex.net/sftp.net/download.aspx
4) Try to upload the same file using other client such as FileZilla or WinSCP or Bitvise SSH client.
If you are able to upload file successfully using other client, you should be able to upload using Rebex as well. In this case, please send us log from the other client so we can compare differences.
5) Update server to the latest version.
From the log file I can see that the server version is 'SSH-2.0-Serv-U_10.4.0.0'.
On Vendors page is written: Serv-U 10.4.0.0 was released January 25, 2011.
The current version is: Serv-U 15.1.5 was released February 13, 2017.
Please keep queuing disabled (client.Settings.DisableTransferQueue = true), it seems it has small positive impact to the transfer.