Welcome Schnippsche, in response to your questions ...
I am not aware of such service features in the current version of Mp3tag.
There are some threads in the forum regarding "Bad ID3v2".
ID3v2: !BAD
Bei Anzeige "(BAD ID3v2)" Korrektur möglich?
... and more ...
https://community.mp3tag.de/search?q=Bad%20ID3v2
I looks like you have to delete the entire defective tag using Mp3tag's remove tag feature.
You can also try to use other service tools, like mp3val or foobar2000, but I do not know how they work on a totally defective tag.
DD.20110816.0758.CEST