My problems are partly solved. I have changed the OS to Volumio. Actually this software acts as an UPnP renderer. If you use Linn Kazoo as a control point (on iOS or MacOS) it works perfect with all files AND metadata including album covers (if you own a UPnP server). Will try if max2play works the same way. Disadvantage is that Qobuz is not working so far in this environment. Seems that I have to setup the MinimServer on my NAS or other RasPi for this purpose.