Update Library (from tags) would be reading the tags from the media files on disk. This seems to imply MC is having trouble reading some files.
The tags are stored in the Library (MC's database). They are read when files are imported into MC. The tags are also written back out to the media files when you change a file's properties in MC (again, presuming the default setting of "Update Tags When File Info Changes"). If media is being analyzed when imported, or cover art looked-up/downloaded, this info is written out to the files (e.g. gain values, etc.).
So, check the permissions of the files and folders on the NAS, ensuring the the MC user can read/write everything from the base folder MC is using (e.g. as configured in Tools > Import... > Configure Auto-Import, or that you've manually imported).
Also, create a smartlist (under Playlists), and place the value below in the Import/Export box:
~d=b
Do you have files there?