Sometimes I stumbe upon mp3 files that can't be changed by Mp3tag.
I handle such files by foobar2000 (v0.8.3).
After processing in foobar2000 files become changable by Mp3tag too.
I hid TAG column years ago, so I didn't see that problem is in Ape tags.
I supposed that unchecking "Write Ape2 tags" and checking "Remove Ape tags" would be enough to clear Ape tags while saving changes.
Seems to be separate "Remove tag" action is unavoidable.
Not very intuitive, imho, but at least now I know how to use Mp3tag properly )
P.S. Please, remove this post if it isn't fit this subforum.
Didn't find how to do it.