Looking at the logs I found this:
07-16 12:11:36.578 i: [FF] Video decoder doesn’t support ac3 (audio/ac3).
07-16 12:11:36.580 i: [FF] Video decoder doesn’t support h264 (video/avc).
I just updated my FireTV 4k to the latest firmware (6.2.7.3) and also updated Plex to the latest version (kepler-armv7a-amazon-fireExo-8.5.0.19697-bdf62378).
AND ALL MY VIDEOS ARE PLAYING AGAIN !!! YEAH!!!
Thank you soo much!
I don’t know whos effort it was. But I don’t care. Finally I can enjoy my whole movie collection again.