The command was:
beet move -c -p -d ~/ogg -a Language Transfer
And the output was something like this
/home/juh/beet/00.1.mps --> /home/juh/ogg/00.mp3
/home/juh/beet/00.2.mps --> /home/juh/ogg/00.mp3
/home/juh/beet/00.3.mps --> /home/juh/ogg/00.mp3
/home/juh/beet/00.4.mps --> /home/juh/ogg/00.mp3
In the meantime I solved this problem by providing a title before importing the files.
All these files got these strange filenames because the track number field and the title field were empty when I imported the files to beet.
If you cannot reproduce the error, I can try to reimport similar files and see if the problem still occurs.