Terminated by user. Copying file fatally failed. Copying files to remote side failed

My .net app based on winscp stopped working. While uploading a bigger file, I get

WinSCP.SessionRemoteException
  HResult=0x80131500
  Nachricht = Terminated by user.
Copying file 'test.mp4' fatally failed.
Copying files to remote side failed.
  Quelle = WinSCPnet
  Stapelüberwachung:
   bei WinSCP.OperationResultBase.Check()
   bei Hochladen.Form1.hochladenscp(String file) in Form1.cs: Zeile81
   bei Hochladen.Form1.<>c__DisplayClass10_0.<werfen>b__0() in Form1.cs: Zeile175
   bei System.Threading.ThreadHelper.ThreadStart_Context(Object state)
   bei System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   bei System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   bei System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   bei System.Threading.ThreadHelper.ThreadStart()

And it is pointing to transferResult.Check();, saying

WinSCP.SessionRemoteException: Terminated by user.
Copying file 'test.mp4' fatally failed.
Copying files to remote side failed.

Scp exists on the server, just like before, the scp password is right, just like the ssh-key.

Here the end of the logs

. 2018-10-19 13:00:37.119 Binary transfer mode selected.
> 2018-10-19 13:00:37.120 T1539559055 0 1539946831 0
. 2018-10-19 13:00:37.151 SCP remote side confirmation (0)
> 2018-10-19 13:00:37.151 C0644 1487483772 WeitWeg.mp4
. 2018-10-19 13:00:37.195 SCP remote side confirmation (0)
. 2018-10-19 13:00:37.195 Sending BINARY data (first block, 32768 bytes)
. 2018-10-19 13:01:06.560 Waiting for dispatching send buffer timed out, asking user what to do.
. 2018-10-19 13:01:06.560 Asking user:
. 2018-10-19 13:01:06.560 **Host is not communicating for 15 seconds.
. 2018-10-19 13:01:06.560 
. 2018-10-19 13:01:06.560 Wait for another 15 seconds?** ()
< 2018-10-19 13:01:06.560 Script: Host is not communicating for more than 15 seconds.
< 2018-10-19 13:01:06.560 Still waiting...
< 2018-10-19 13:01:06.560 Note: If the problem repeats, try turning off 'Optimize connection buffer size'.
< 2018-10-19 13:01:06.560 Warning: Aborting this operation will close connection!
. 2018-10-19 13:02:06.162 Attempt to close connection due to fatal exception:
* 2018-10-19 13:02:06.162 **Terminated by user.**
. 2018-10-19 13:02:06.162 Closing connection.
. 2018-10-19 13:02:06.162 Sending special code: 12
. 2018-10-19 13:02:06.162 Sent EOF message
< 2018-10-19 13:02:06.166 Script: Terminated by user.
< 2018-10-19 13:02:06.166 Copying file 'test.mp4' fatally failed.

< 2018-10-19 13:02:06.166 Copying files to remote side failed.

What could be the solution?