About GoodSync:
In my case, I was getting intermittent errors too.
I found the reason. GoodSync implements SMB2 and QNAP works with SMB3.
I only needed SMB to work with GoodSync, so I decided to enter in QNAP Control Panel, Network Settings, Win/Mac/NFS, Windows Advanced Options, SMB Max Version and change it to 2.0.
Now, GoodSync and QNAP uses SMB2 and the problem disappered.