I have just done a test as follows bearing in mind that in my case the tagging errors are only happening with MP3 files and not Flac or Ogg.:
I took an album in Flac format already in my library and confirmed that I could perform tagging operations on it without any erors. I then selected the album and went to 'Library Tools --> Convert Format' and converted it to MP3. I then imported that MP3 conversion into MC and then attempted to alter the tags. Once again I get the tagging errors when attempting any tagging operation. However, if I then check the tags with MP3Tag I can confirm that the tag edits I made did in fact get written to the file.
Well, then... That
does seem unlikely to be a file pemissions issue on disk.
Can you confirm a few other things:
1. When you did the above-explained test, what conversion settings were you using?
2. Where on disk were the converted files going (specifically)? Same folder as existing, or to a specified location?
3. If specified, is that location also on the NAS? And, does it make any difference if you go to a local disk?
4. You might have addressed this above, but it IS possible that Security software
can cause this kind of issue. I've personally seen it with older versions of Symantec Endpoint Protection and network UNC volumes. Just to rule it in or out, can you repeat the above-described test with any AV application completely uninstalled? You can reinstall it if nothing changes, of course, and if it does fix it, we can probably figure out the cause (or get exclusions set up such that it stops happening).