Thank you for the advice.
The music files won't complete loading, with or without 'library' activated (I tried deactivating library and deleting the mp3tag.db3 file, but it still loads the files then 'spinning blue circle').
I am currently running the integrity checker in Foobar2000, as suggested here:
I think 'Beatunes' is the cause, which I use as well as Mp3tag. I haven't encountered issues using both before, and I've been using Beatunes for over ten years, but this is since performing edits in there.
Hopefully, Foobar2000 can identify the issue(s).