I’m on Windows 10 64bit and was also experiencing this for the past month or so. I also do CPU mining on my Plex server so I thought maybe that was causing issues. Rebooted fresh, nothing running, still received the transcoder error when playing any .mkv local or remote. Noticed that I had a missing update for KB4340917, installed that, and Plex has been stable. Reading on this KB patch I don’t see what is included that would change or fix the transcoder.exe Plex uses, but things definitely seem better.
