We should always at least tell if a problem is solved, or not !
I have a similar problem, but with a Amp2 HifiBerry card, and a WiFi access-point activated. All other configuration is identical. Error message in mpd.log is very simple : „Failed to open Alsa Mixer…“, without anymore specificities. I can change the volume from the Raspberry config tab, but not from YMPD.
Any idea ?