Plex Home Theatre "Unable to Play Media" message appears but file still plays

Plex Players Desktop Players
2

PHT is dead.
OpenPHT is dead.

Switch to Plex For Windows/Mac:

https://www.plex.tv/media-server-downloads/?_ga=2.15809270.1993529163.1575302888-1864473727.1455657109#plex-app

Or, if you like TV View Mode/10 foot remote - Plex Media Player:

Newest Version always at the BOTTOM of that thread.

That should take care of your playback issues (we hope).