Hi Bill.
Thanks for you comments.
Im connecting through internet. The link is very stable and fast (30 MBit, fiber optic). We do have a firewall set up, no proxies and no VPN. I can try syncing from the SMP location, but I highly doubt this is the issue, as my other devices (Android and iOS) connect and sync without any issues or errors.
Sometimes, after I restart the sync process, the error occurs after less than a minute of data transfer. I checked the numbers on the server side, and I set them, for test purposes, very high (timeouts in hours).
Marko