Fedora 34 KDE - display goes offline at login screen

Does KDE use Wayland or Xorg?

Double check and if you are using Wayland, switch to Xorg, because Wayland does not work well with NVIDIA.

Your session can use Xorg while your login screen still may use Wayland. This may cause your problems on the login screen.

In that case you would have to set the default to Xorg.

But after all, I’m not so familiar with KDE.
Someone else may help better with that.