"...anybody using the M-One as UPnP device without problems?"
After some early poor reliability, which a couple firmware reinstalls & reboots seemed to fix, my M-100 has been very reliable on the network as a UPnP renderer. I use the Micromega Android app to control volume and input sources only - other UPnP control point apps are much better, especially the BubbleUPnP android app (sadly not an option for iOS). On Mac OS I use upplay as control point app, also quite good if not rough in its ux implementation.
Direct-message me for a contact at Micromega en Boissy-Saint-Léger (he’ll connect you with a tech support person who can help).