Chromecast Ultra and 2nd Gen - Error code: h4 (Not Supported)

I’ve got 2nd Gen and been having h4 error for a while now. It only affects certain mkv blu ray rips and not all. No idea what is causing the issue.

I’m running Plex on Synology.
I tried staying playback in mobile and then casting but that doesn’t fix the Chromecast problem.

Any ideas?