I found something. Removing and reinstalling the libspotify12 package fixes the problem for me:
apt-get -y remove libspotify12 && apt-get install libspotify12 && service mopidy restart
hope it lasts, so I can create the final version of MusicBox 0.5.1. It’s the way I fixed a strange bug some time ago too…
I commented out the cache_dir setting in the config too.