Here is an old piece of software that is irrelevant by itself, but it is useful to replicate an Mp3tag bug.
So basically if you use this old software to convert any FLAC file to WAV file, the resulting WAV file cannot be edited by Mp3tag.
Can you replicate this Florian, can it be of help to diagnose why the resulting .wav file cannot be edited by Mp3tag? It's a problem I'm having on my system. Here's the converter that creates a .wav file that causes this problem: