I had exactly the same issue at the beginning of this month, it is due to a recent change by Plex having to do with security protocols of older TVs. It is actually very easy to fix as already reported in some other posts:
In plex on the desktop, in Settings > Network, make sure ‘Secure connections’ is set to ‘preferred’ and not ‘required’ and enter the IP addresses of your TVs in ‘List of IP addresses and networks that are allowed without auth’.
In Plex on the TV, you want to make sure that in Preferences > System, ‘Use secure connections’ is off.
You also need to enter your PMS IP address (your desktop PC or whatever you’re using) in Plex Media Servers (see the manual if you’re not sure how to do that).
Then of course you should not select ‘sign in to your plex account’ since you will not be connected to that account anymore, but instead just connected locally to your server (so that is also the potential - makes no difference for the way I use Plex - downside).
I got the same error with my UE46ES5700. I tried to uninstall and then reinstall Plex but it was not found in Appstore anymore. I think the TV has had its best years soon. That’s why I’ve been using a Chromecast now for a longer time. I can only recommend this to anyone with older models. Simple and fast. Low investment.
I tip my hat, it works, unbelievable. Thank you Boup, you are MVP. I wish you all good in life and more!
I tried so many things, other programs, private servers whatnot, hdmi cable, chromecast and stuff. This very simple solution made my day! I even made an own plex topic where i was met with negative attitudes like just buy a new TV or whatever. A solution like this was exactly what i wanted but nobody even bothered there.