I tried testing a movie from my home server in plex through firefox and repeatedly got this message, even after reloading.
I knew that they had paywalled the apps on mobile and streaming from outside the network but now they have also blocked watching your own movies through your own hardware.
I do get the point that making software should be able to sustain people but I dont see the move of plex as a fair thing to do. Yes, they have made great software but taking your home server hostage feels like the wrong move.
Even a pop up that says "we need you to donate please" would have been fine. make it pop up before every movie, play donation ads before any movie but straight up disabling the app is kinda cruel.
Anyway, i have switched to jellyfin and it is insanely good. please give it a try. you can run it alongside plex with not issues (at least i had none) and compare the two.
In any case, good luck. Let me know if you need help.
Seriously. I hate when people assume default settings are the only option. You don't even need a Plex account to set up Plex. It will just be less seamless and user friendly. Never adopt the server, configure these via localhost (ssh tunnel works) and then set up your networking. Don't even need to update it, it will run for as long as the database stays stable. Which should be years or more.