Could you share some logs?
Either from the /dev page using the livelog feature or via ssh.
# Volumio logs
journalctl -f -o cat
# vollibrespot logs
journalctl -f -o cat -u volspotconnect2.service
Note you might need sudo before these commands depending on the version of Volumio your running
I would guess it fails to figure out the correct device/mixer and hence fails.