Thanks for your help.
Same results with VDR 2.6.4 while using only the following plugins:
streamdev-client, svdrpservice, softhddrm and mpv.
Without skindesigner no dump but osd does not show up. With skindesigner VDR crashes.
I have no live Plugin or KODI installed. Using mpv from command line works fine (that's also the case from mpv plugin as long as osd is not opened.
Bootloader is Grub2 with a fixed EDID from my av receiver:
GRUB_CMDLINE_LINUX="video=HDMI-A-1:3840x2160@50D drm.edid_firmware=HDMI-A-1:edid/edid.bin"
I will dig deeper into the mpv plugin on the weekend.