Turns out the problem was something else. I noticed that the MP3Tag playlist file was in ANSI and the playlist file from MB in UTF-8, both of them being an "Extended M3U".
Apparently the car radio can not handle song names with an umlaut, (for example Über Den Wolken.flac) if UTF-8 is used. It is not a problem if ANSI is used. I have no idea why, but it works if i just save the file created by MB in ANSI.
The file created by MB works fine on all my other media players, (Kodi, phone, PC, tablet etc.)