Buffer Error

This one vendor we are having issue with uploading files to. We are able to download files with no issues. When we try to upload a file it goes extremely slow and timesout. We are able to upload with Filezilla and use SFTP from the Linux terminal. I have attached a portion of the log below.

. 2017-05-24 14:26:13.040 There are 32832 bytes remaining in the send buffer
. 2017-05-24 14:26:13.040 There are 32832 bytes remaining in the send buffer, need to send at least another 64 bytes
. 2017-05-24 14:26:13.140 There are 32832 bytes remaining in the send buffer
. 2017-05-24 14:26:18.923 There are 32832 bytes remaining in the send buffer, need to send at least another 64 bytes
. 2017-05-24 14:26:19.026 There are 32832 bytes remaining in the send buffer
. 2017-05-24 14:26:19.026 Waiting for dispatching send buffer timed out, asking user what to do.
. 2017-05-24 14:26:19.026 Asking user:
. 2017-05-24 14:26:19.026 **Host is not communicating for 15 seconds.
. 2017-05-24 14:26:19.027
. 2017-05-24 14:26:19.027 Wait for another 15 seconds?** ()
. 2017-05-24 14:26:19.287 Session upkeep
. 2017-05-24 14:26:19.791 Session upkeep
. 2017-05-24 14:26:20.301 Session upkeep
. 2017-05-24 14:26:20.803 Session upkeep
. 2017-05-24 14:26:21.315 Session upkeep
. 2017-05-24 14:26:21.518 Attempt to close connection due to fatal exception:
* 2017-05-24 14:26:21.519 **Terminated by user.**
. 2017-05-24 14:26:21.519 Closing connection.
. 2017-05-24 14:26:21.519 Sending special code: 12
. 2017-05-24 14:26:21.519 Sent EOF message
. 2017-05-24 14:26:21.520 Connection was lost, asking what to do.
. 2017-05-24 14:26:21.520 Asking user:
. 2017-05-24 14:26:21.520 Terminated by user. ()
* 2017-05-24 14:26:22.692 (ESshFatal) **Terminated by user.**
* 2017-05-24 14:26:22.692 Copying files to remote side failed.

Please attach a full log file using WinSCP 5.10 beta.

Attaching log file.

Thanks. And sorry for my late answer.
Please increase logging level to Debug 2.

Attaching Log file

I do not see any problem on WinSCP side. The server seems to stop responding, as soon as the transfer starts.