as you mention, restarting mpd works.
However, a reboot did not work before I put your suggestion in practice: I rebooted quite a few times and each time the webui did not respond (ssh worked instead).
Now, when I reboot it works.
So I think that either mpd was not started at boot (due to some other failure), or starting mpd failed for some reason at every boot.
How can I check that /etc/init.d/mpd restart was in fact called during boot? I mean: any log I can check that?
Where is /etc/init.d/mpd restart invoked from in the init scripts?
I’m wondering whether the issue might be related to any unclean/abrupt disconnection of the RPi: there being no UI apart from the webui, it might not be possible to know exactly when the RPi is actually off (yeah, all leds of the Rpi go off apart from the red power led, but I think that after all leds are off, still the RPi might be in the process of closing down) .