Thanks for the suggestion. I installed Process Explorer but when running it, I did not understand very much what it displayed. But I have come along with a solution, which popped up in my mind. I have two LAN cards in my desktop, both attached via LAN cable to the switch in my home network, both with static IP-addresses. Usually one interface is deactivated, but I had activated both thinking that this might speed up transfer between desktop and NAS. Apparently that only screwed up things. I disconnected one LAN cable, rebooted and now Goodsync transfers all files not transferred before, also estimated sync time about 6 hours compared to several days with both LAN connections active. Now only about 45 min left and no error message yet...