Video I could play the other day, I can't play today

The other day, I streamed a video on a Fire TV Stick without any problems. Today, the video won’t play at all. It just buffers (yellow circle going round and round). The only thing that has changed is that I signed up for Plex Pass. I was able to play other videos, although there was some buffering for a few seconds.
It is direct playing as MKV.

Any help appreciated.

Server Version#: Version 1.42.2.10156
Player Version#:
<If providing server logs please do NOT turn on verbose logging, only debug logging should be enabled>

Update: When I turn off hardware acceleration, the problem doesn’t occur.
I’m using an RTX 3060 Ti, which I thought should be sufficient, no?