Black screen on Plasma Wayland but not X11

Hi,
with my first user name, created during installation of Leap 16, I can only log in to the KDE/X11 desktop, but not the KDE/Plasma Wayland desktop. This results in a black screen, then a reboot follows.
However, with newly created users, there is no problem. Even, with the new user I created with the System Settings App in the X11 desktop I can log into the Plasma/Wayland desktop (at least, the desktop opens, I don’t really see a difference to the X11 desktop and echo $XDG_CURRENT_DESKTOP gives “KDE” for both desktops).
So, I guess, something’s wrong with the user settings of my first login. I only noticed this a few weeks after the Leap 16 installation, because X11 is the default desktop. I didn’t do too many things with the first user yet, but I activated some Widgets, maybe they are peculiar to one desktop version.
Now, I wonder, which are the places of settings, that I have to delete, to get my first user a “virgin” user, again, with the possibility of using Plasma/Wayland?
I see .config, with many plasma related entries, .local/sharewith kded6/ and some other entries that might be connected to the desktop.
Or would you advice to rather setup a completely new user?
Thanks for advice.

Thank you. That other thread gave me the idea of trying another user, but I couldn’t understand from it, what configuration files are the problem for Plasma Wayland. Now, I read it again and I saw the suggestion of setting the theme back. Unfortunately, that didn’t help in my case.

I’ve set the general theme in Colors&Themes back to openSUSE and the wallpaper to openSUSE default (the yellowish thing). “Plasma Style” is also openSUSE. I don’t think I have changed anything else. I even set back the “Power Management” to “Defaults”.

But I still get the black screen and a re-boot.

Any ideas?