A GNOME login screen might not start and display Oh no! Something has gone wrong message instead. This only happens if you have installed video acceleration codecs from RPMFusion. (There might be other reasons why you see this screen, but those are not covered in this topic).
GNOME login screen fails when important Mesa subpackages (like
mesa-libEGL) have a different version than the main driver package
mesa-dri-drivers. This problem should be prevented once you have mesa-23.0.3 or later version installed. However, when you have
vdpau) installed from the RPMFusion third-party repository, it might block upgrading all Mesa packages to 23.0.3, because the
-freeworld packages have an older version. The latest mesa-freeworld packages are already built, but RPMFusion maintainers push the updates infrequently, and they haven’t pushed it to the updates repository yet.
Reddit: System got totally borked after upgrading to mesa 20.0.3 because of mesa freeworld
If your login screen is broken, press
Ctrl+Alt+F2 to switch to a tty2 text terminal, then log in. Afterwards, uninstall all mesa-freeworld packages:
sudo dnf remove 'mesa-*-freeworld'
Now fully update your Mesa:
sudo dnf update 'mesa*'
And now install official Fedora video-acceleration drivers:
sudo dnf install mesa-va-drivers mesa-vdpau-drivers
Stay away from installing mesa-freeworld packages from RPMFusion, until the necessary updates have been pushed to stable updates - in this case,
vdpau) 23.0.3 or later. Even then, please be aware that installing core system packages from a third-party repository poses a system stability risk.
You can discuss this topic here.