Isn't the server under a heavy load, during those 15 minutes? Did you try setting a longer
I'm running a 8 core Server running at 5% CPU when error occurred.
SessionOptions.TimeoutInMilliseconds set to 2 minutes, I believe that delay is the establishment of a session would expect to get a WinSCP.SessionRemoteException. Not really sure when System.TimeoutException is thrown.
I modified my send logic to implement a return loop (Circuit Breaker Pattern); Retries continue until it finally runs after 10 minutes. Bottom line, can code around the issue, still not sure why the timeout occurs. SFTP Site is up and running, can open the WINSCP UI and connect fine (no error).