I’ve just installed Leap 15.4 and found exactly the same problem described by marvo22. It’s undoubtedly caused by a bug in SDDM because other display managers appear to work. I haven’t surveyed the numerous posts on this topic, but can possibly add the following.
When SDDM gets into this state, it usually seems to apply to a particular user, so one user gets a blank screen while others work normally. However it’s not user-dependent, and sometimes more than one login user can be affected concurrently, including the root user.
If the user does nothing (for ~5 minutes in my case?) after logging in, they will eventually be rewarded by a normal Plasma session with their usual wallpaper, icon arrangements, task bar, etc. It’s as though some timeout occurs, or maybe the screen energy-saving process kicks in and moving the mouse then brings SDDM back to normal operation. Or something…
In another post at https://forums.opensuse.org/t/how-to-change-display-manager-from-sddm-to-lightdm-on-plasma-tumbleweed/164389/11 deano_ferrari suggests installing LightDM:
There is no need to configure LightDM. In an effort to assist I just installed the LightDM display manager and selected it with ‘update-alternatives’. At the login screen it is possible to choose ‘Plasma (X11)’ or Plasma (Wayland)’ at top right of screen. The session type should remain as chosen following a reboot. The previous session state is stored in /var/lib/lightdm/.cache/lightdm-gtk-greeter/state.
I’ll give that a go, though it may only work in Tumbleweed…
Is this problem fixed in 15.5?