I checked that thread and downloaded the image from it but it doesn’t fix it for me. Also, after more testing I found out it has only to do with 4k itself and not HDR specific. Actually I’ve been able to get my BBC earth 4K/HDR playing by whitelisting 3840 × 2160 @ 25Hz. But as soon as I stop the playback my screen get distorted.
For now LibreELEC 9.0.0 unfortunately is the only distro which plays all my files with correct video, including 4k and HDR. But it has two major drawbacks which is no official support and somehow it does not playback MP2 audio (from satellite TV).
So I hope this will get fixed some time.
EDIT: After some further study I now have my MKV files playing fine. For that I had to whitelist all the 4K resolutions. Still don’t understand why that is necessary only with CoreELEC. Anyway, my 4K live TV satellite channels still give audio without video. Video is visible in PiP preview but “No signal” as soon as it goes full screen.