It's also possible that the files are damaged or otherwise malformed. Mp3tag refuses to write files with internal errors to prevent further damage (happens most often with WAV and MP4).
Regarding the error messages: I've improved that with current v1.2.1 (49).
Fantastic, I will bear in mind about possibly damaged files, and other programs that might have been using the files. I was ripping some of my own CDs to my NAS, and had Plex running, which might be the reason. I am re-ripping the 'offending' CD and will have another go.
Well, that was pretty straightforward! I quit Plex, and reattached the sleeve art to my files - no errors. Incidentally, I noticed that I had previously attached other versions of the sleeve art to some of my files, so I deleted all sleeve art from every file and re-attached only the sleeve art I want.
I recall someone recommending that I make all my sleeve art files 400x400pixels. Some of the troublesome ones had previously been more that 400x400. Could that be why mp3tag doesn't like them?
After I had started Plex again, I noticed (after I ran the Plex process, 'Scan Library Files') that Plex still wasn't recognising am few albums correctly. What has greater priority, Plex or mp3tag? Sorry if this is covered in the help section.
From what I've understood from your previous posts, you're running Plex on a NAS so it should just reload the modified data if you're scanning the files.
Are the files already listed in Plex but with the old information or are they not showing up at all?