Error code 58 during file copy

[bortoman]

Hi,
i'm using FreeFileSync for a while now and it always worked great, but now i'm getting the following error:

Capture1.PNG (11.72 KiB) Viewed 249 times

File comparison works fine, but after that i'm not able to synchronize!
I get this error for every file, every time i retry...

I can have full access to the NAS, and haven't touched it's configuration.
Trying to use IP-address doesn't help. Trying to synchronize two local folders works, what could it be?