I am experiencing the same issue since the update that contained the new player in it. My server is remote and was working just fine prior to the recent update.
I have determined, through frustrating trial and error, that if I close Plex on my Shield TV (via double home button press) and open it 3 times in a row, on the third time everything seems to work fine.
This is consistent and the household method for watching anything on Plex now is “try opening and closing the app 3 times.”
