"!BAD ID3v2" - how to restore the tag information?

I have about two thousand files with bad tags. When I look at these files in MP3Tag, the tag field for these files says "!BAD ID3v2". When I look at these files in musicbrainz, and I change the comment field, all of the “missing” mp3 data magically reappears. When I do this in MP3Tag, however, nothing happens. How can I fix these tags in MP3Tag?

See here for more hints on how to get files back from the state of bad tags:

But depending on the extent of the damage, it could be that the info is lost.
You could experiment a little and see if V1 tags are still readable. See Tools>Options>Tags>Mpeg and set ony V1 to read, all other off

Thanks for taking the time to reply. I downloaded MP3diags, but the program, according to itself, was last updated in 1991, that is, 31 years ago. I'm amazed it still executes. I don't want to use this as it has not been updated in so long. It even looks ancient.

Also, the info isn't lost (as I stated in the original post), it is retrievable using musicbrainz. I wanted to know if the info was retrievable using MP3Tag.

If MP3tag considers the tag to be bad, then that's it as far as MP3tag is concerned.
I wonder what MP3tag and the other programs say about the tags that have been rewritten with Musicbrainz ...

Well, the MP3 format also has not changed for a long time.

Not sure where you got "1991", but the last release version was actually from 2016-07-29 (https://sourceforge.net/projects/mp3diags/files/ . You might also try MP3val which though older (2009) is simpler to use and has worked well for me (https://sourceforge.net/projects/mp3val/files/mp3val-bundle/ .