After some investigation and grepping I've located the bug in mp3tag.cfg.
The problem results from deleted! mapping entries which are still present in mp3tag.cfg - but invisible (and therefore undeletabe) in the user-defined field mappings window.
Linux grep found two entries:
APEv2 ... REPLAYGAIN_ALBUM_GAIN ... replaygain_album_gain
APEv2 ... REPLAYGAIN_ALBUM_PEAK ... replaygain_album_peak
Some times ago I tried out mapping of APE tags but deleted them after. But they aren't deleted completely - they disappear from mapping window but keep still active in config file. Strange.
The mp3tag.cfg is backed up. If a developer wants to do some tests...
I would then throw away the old mp3tag.cfg.
A new one gets written as soon as you open (and close) Mp3tag again.
You will losoe your custom settings though and various input histories.
Actions, web sources and exports are kept.