Recognized that mp3 files tagged with Mp3tag since v.3.00 and higher get a corrupted header, when using images as cover which where optimized for web (e.g. saving as web optimized file with IrfanView).
After tagging the mp3 files, they can no longer be opened in Audacity for editing. Audacity shows an error message "mp3 decoding error (reserved header layer value)".
When removing the cover with Mp3Tag v3.0x the mp3 files can then be opened in Audacity for editing without any problems again.