I was having this problem with Windows Server 2016 and FM 17 Server.
I switched the OS on the Server back to 2012 R2 and upgraded to FM 18 while I was at it. And that resolved the issue.
I think the crux of the issue might be that on Windoes Server 2016 it was trying to connect to Port 16000 during the upload to server process. Whereas on Windows 2012 it was connecting to Port 443 without any issues.
Anyways, I'm not saying that's the issue. Just a hunch. But going back to Win Server 2012 solved my issue.