Thanks for all your help! I have a working KaOS system again!
@peter-hb-1955: i have a different board without this option so i could not use your tip. I tried out all possible SATA options in my BIOS, but it didn't solve the problem.
@demm: you were right, downgrading to the previous sddm package solved the problem. But i had to copy over the previous sddm package file from my laptop, so it was just luck to have a second KaOS-2017-06 based system lying around. Others, like @wolf51, maybe are not so lucky, so what can he do? I did not find any old package versions in the KaOS package archive.
And now I have a different problem, which is much smaller but annoying. Everytime i login to ma system octopi wants to re-upgrade my downgraded sddm package. I am also forced to do that before installing any additional package with octopi. And after that before shutting down or restarting i always have to downgrade sddm again to make my next system startup work. Is there a way to tell octopi just to ignore the old sddm package?