Stephan-P I have now installed all the apps from the G play store. But doing the same with Android Auto contradicts the GOS direction of shoving these things all into the sandbox. If GOS has Android Auto in the sandbox, it should work across the board. OK, voice input doesn't work in GOS either, but that's probably not linked to the music player, is it?
Anyway, I tried to get Android Auto from play store. I could deinstall it from there, but not install. It seems that it is only possible to install it from GOS store. That would make sence, right?
After clicking on "update" in play store, music player began to play more than a second - about 6 sec - , but then decided to quit.
It is still completely unclear what causes it or what I failed to set / unset.
It could even be a bug that came with last system update. However, then I would not be the only one having this issue.