Do you experience problems during downloading?
Check your connection in Status and Interface settings window.
Use Test Server in Config > Servers.
We will probably ask you to do a test using only basic settings.
Do you experience problems during repair or unpacking?
Enable +Debug logging in the Status and Interface settings window and share the relevant parts of the log here using [ code ] sections.
I think should try with the same long file names SABnzbd uses as the problem might be in:
your NAS refusing long names
your NAS refusing names with spaces
your NAS refusing subdirectories
your NAS refusing special characters
your NAS refusing copy / write within a subdirectory owned by another user
etc etc