Surprisingly Plex now finally works with my MCH for the first time in months. Both on my Mac and on iOS-app. Looks like they can not make it run for everybody at the same time.
update: But plex still can not be accessed to from outside my network. So it is still useless.
I will continue to use the FLACBOX App for Musicstreaming instead. The “user experience” is thousend times better than plex.