There is no Firewall in the middle. It all goes over a VPN (from source to target server) where all ports are passing through and the Windows Firewall on both servers is turned off.
It might be a setting of the FileZilla FTP Server I'm running on the target server, but if I use FileZilla Client on the source server to transfer the same files to the remote, it all works fine.
The good thing, though, is that next time BestSync run's the scheduled task, it corrects the zero-seized files on the target. But still that's a problem cause until the schedule re-runs there are zero-sized files on the target.
Any other thoughts?