Thanks for your reply and sorry for the belated answer.
The notification mail got put into the Spam folder.
It seems the fallback to SW decode doesn't work for me.
To get any kind of video output with h.264 video (1080p) and FLAC audio I had to increase the allocated RAM for GPU in the config.txt to at least 128MB (it's on 256MB now) but with pink/purple fragments and it sticks to PRIME DRM HW decoding.
Manually setting the decoding to software did the trick for those kinds of videos.
That is a workaround.
Maybe a clean install of libreelec might do the trick here. I do faintly recall noticing something like this with the LE 9.x versions.
Thanks again for your explanation and redirection to the Kodi forum regarding the subtitles.
Greetings and regards
IG