yesterday I did a "zypper dup" for my Tumbleweed installation on a laptop and
it ran without errors. 

After reboot Terminal 7 remains dark with a cursor in the top left corner. Last
year I already had the same problem and solved it by a complete new
installation of Tumbleweed. But obviously the problem still exists.

What I observed:

- sddm is running together with a greeter process

- screen is dark, but after entering the password everything seems to work fine
"in the darkroom" as can be seen in another terminal: the user session is
started correctly, all user processes and initial windows are seen in the
process list, but only the cursor is displayed on a dark screen

- changing to gdm with update-alternatives and restarting the display-manager
service: Terminal 7 shows the login screen and normal login is possible ==> X11
and graphics driver are working fine obviously

- changing back to sddm : dark screen on Terminal 7 is back again!

- removing sddm-caches in /var/lib and user-home doesn't help

- reinstallation of sddm doesn't help

- Xorg log shows an error message from sddm: the file /run/sddm/\{.......\}
can't be opened, but an ls in another terminal shows, that it is created and
destroyed with starting and stopping the display-manager service.