I finally took some time and figured this issue out. The Problem was the „Hide ‚All channels‘ group“ setting. It was enabled like it was on my MP1 setup, but it seems to be different in MP2.
But the import thing is: It is working now!
Thanks for all your efforts.
„Hide all channels“ can be defined in the client settings under ‚TV‘.
There should be no difference if this is enabled or not. If there is for you, it could be some leftover from MP1. We haven’t heard of any problem in this regard yet and you are the first case. Can you explain in more detail? Where was it enabled?