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.
This is one change (which isn't the cause of OP's problem) that they announced months ago. I've been using it for well over a decade and while I have had major issues with it in the past going so far as to setup Emby and buying a lifetime license for that, I would hardly say that they're "constantly changing allowed abilities."
Most people's issue with them is that they focus too much on adding new stuff that nobody asked for while ignoring longtime bugs. I can't recall a time they've ever locked anything behind a paywall that wasn't a brand new feature prior to this.