From one day to another, my music player Amarok stopped working. It gave me the following notice:

A new user was able to play MP3 files with Amarok though, so something was messed up with my own configuration. It didn't turn out to be the Amarok settings. But luckily, things started to work again after replacing an older revision of the ~/.xine folder (hooray for rdiff-backup). Removing ~/.xine altogether should do the trick as well.

It seems that some process deleted a whole bunch of plugins from ~/.xine/catalog.cache. But I don't know how that could happen.