I am having this issue all of a sudden as well. Must have been from a recent update. All of my videos, no matter the quality, size, resolution etc. are not playable by Plex app on my Roku TV or plex desktop. Says the server isn’t powerful enough.
I’m running a windows server on Pepper Jobs mini PC. Definitely powerful enough.
…and yet when I play from my iPhone there are absolutely no issues in playback. So weird since it’s all originating from the same server anyway…it seems to be device based.