Basically if the MKV file has a Data Rate it will crash the Amazon Fire TV App and also on the Sony TV Plex App.
However, the MKV file will play okay through Windows Plex Media Player. Its certainly a major bug which I hope the plex developers will fix. Let me know if you want a large sample MKV file with a Data Rate of 0kps.
Please provide the “Get Info” and View XML results? There is no way of knowing what’s going on from your OP. Fix what? What’s broken?
Let’s start there and see what Plex thinks it’s looking at.
Next, if you have another tool to show what should be in the file (media info, ffmpeg, something) that output would be equally helpful as a comparison.
Edit: If, after you attempt to play the file, you wait 30 seconds and then go to Settings - Server - Help - Download Logs and attach that ZIP file here, it will also help us by showing what PMS is doing at the time of failure.