Dear Eumel, Dear Hudak,
I've been on a business trip the last two weeks.
Last weekend I tested your suggestions, but it didn't help. I changed the HDMI Port of the Beamer, even changed the HDMI Cable, but it didn't help.
Sadly I didn't have a HDMI-DVI Converter, so I will try this at a later point. Will 4k HDR playback work with the adaptor?
At last I booted up my old RPi3 with Libreelec. 3D Playback (Frame-Packing) without a flaw and no flickering glasses.
Kind Regards
DrDoOm