What is special about those MKV files? Do they create an ffmpeg log when you play them? Can you kindly share it with us.
If it isn't creating an ffmpeg log it is trying to direct play but the device is actively refusing? O_o;;? hard to tell if it isn't creating an ffmpeg log other than maybe ffmpeg isn't starting when it should. Can you share your Emby server log with us so we can check that too?
I attached a log to my first post but here are others that were just created:
These were generated while the froze bar was on the screen:
E-Server-10-10-before forced exit.txt 293.73KB
E-fmpegLog-10-10-before forced exit.txt 1.46MB
These were captured after I exited the app on my Ultra:
E-Server-10-10-after forced exit.txt 616.13KB
E-fmpeg-10-10-after forced exit.txt 1.59MB
I hope those are of some help. Again note that if I turn off HW Accl then the files play fine and they also play fine if I use my Server on my Shield.
Good luck. I understood almost nothing in the logs but I have never learned to speak and read Emby-a-Log. It is a more difficult language than Dutch or Japanese.