I had the same problem after updating my Pi4 to buster!
I fixed it by changing the ownership of the /tmp/snapfifo file from “snapserver:snapserver” to the “mopidy”-user:
sudo chown -R mopidy /tmp/snapfifo
And restart the mopdiy-service as usual
sudo systemctl restart mopidy.service
This fixed it for me!
BTW here is my audio output entry (im using icecast, snapcast-server and client and mopidy [with iris])
output = tee name=t ! queue ! audioresample ! queue ! audioconvert ! audio/x-raw,rate=48000,channels=2,format=S16LE ! wavenc ! filesink location=/tmp/snapfifo
Btw. you can run this command in a second terminal for debugging. This was a big help for me:
sudo journalctl -u mopidy