Issue with Streaming Plex

Hi guys

I've never delved into the realms of Media servers etc and have recently set up on a spare computer a Plex Media Server that is in its natiuvce install config. I have two TV's, both connected via a wired LAN and are connected via a switch. Both TV's have the latest firmware.

My issue is pretty simple.

I can watch any movie on my Panasonic TH-P55VT50Z that is served via Plex

I get an error message on my Panasonic TH-P42GT20Z when I try to watch the same movies on this TV.

The message is 'File Not Supported'

I can see the server, list the movies, just can't play them. Any idea why not?