The Xorg log file inidicates a problem with modsetting of the card. During upgrade I was running out of disk space. The failure was on a package that later installed fine after freeing disk space (don’t remember exactly but it was not X related). Otherwise the upgrade seemed to have worked fine.
uname -a
Linux localhost 6.2.12-1-default #1 SMP PREEMPT_DYNAMIC Thu Apr 20 11:01:10 UTC 2023 (eb3255d) x86_64 x86_64 x86_64 GNU/Linux
Note that the laptop was connected to a docking station for the sake of providing network access. The extra monitors are usually not present.
Another small note: Upon first reboot, the boot menu still offered a 5.x kernel and 15.4 boot entry. X windows came up. When running another zypper up, deletion of the 5.x kernel was suggested. After deletion and another reboot, Tumbleweed and a 6.2 kernel were offered but the X server did no longer start.
Well, I went the route 15.3 → 15.4 → Tumbleweed successfully on another machine. Most likely it was my own fault as I went out of diskspace during the upgrade. I did boot into the rescue system but could not pinpoint the problem, probably kernel related. I ended up moving my home folder to another machine (no separate /home partition), reinstalled and moved back. The machine is now fine (writing from it).