Whenever I stop (stop, not pause) the music, I cannot resume playing anymore. Restarting MPD won't work. The only option is powering off the PI and rebooting it.
Of course the workaround is to always use Pause instead of Stop, but it still is a lot of trouble when I, or friends who are visiting, misclick.

Is it a bug?

For the last week this problem has occured around 4-5 times now. I've controlled RaspyFi mainly with my phone (MPDroid). I pause the song in the morning, when I get back in the afternoon I select a new album to play and it won't play. After that MPDroid is not able to connect with the mpd-server anymore.

Just now it gave an error message when this happened (in Cantata though). The reported error was that MPD was unable to decode the selected file. I checked several albums before Cantata or MPDroid weren't able to connect anymore and I had to reboot the Pi again.

Any idea what is causing this?

Restarted it last night when I got home because of the same problem again. I rebooted the Pi but didn't play any music (I didn't want to wait so I put on an old vinyl).
Same problem this morning. Luckily Cantata does give some feedback if MPD does not work correctly. Error in Cantata: "MPD reported the following error: problems opening audio device.".