This is just for future reference and possibly to help others.
Today I did an upgrade in testing and got, on a later boot, no SDDM login screen. I saw error messages scrolling by before I got TTY9. The last ones repeatedly said that SDDM was started and then stopped immediately. In the logs (`/var/log/kern.log`), I found that SDDM said that `Display server failed to start. Exiting`.
I remembered and found in the APT logs that I had upgraded the kernel, but saw that the NVIDIA kernel module was not recompiled, as it does normally. Not sure why, maybe because it was a minor upgrade only? In any case, reinstalling `nvidia-kernel-dkms` to trigger the recompile did the trick for me: `aptitude reinstall nvidia-kernel-dkms`. The first time booting got me a black screen for some reason, but the second boot (cold one this time), I was greeted by SDDM again.