Comment # 3 on bug 1084629 from Marius Kittler

> Or Mesa, or Qt, or graphics driver, or X...

I excluded other components in the graphical stack because these were/are
working fine, eg. in Plasma session.

> Does this issue still exist?

At least not in my case. I was able to fix the issue by getting rid of NIS.
Apparently it was searching for users via NIS which took sometimes very long.

However, I would still consider this a usability issue in SDDM. It doesn't tell
you what it does - not even via the journalctl log - and just leaves you with a
black screen. There's no loading indication or a way to abort/skip. But this is
likely something for the SDDM upstream issue tracker.