On my desktop, if I change the SDDM theme through System Settings and restart the session, SDDM will fallback to a blue ugly screen like a lightdm screen.
I can fix it, by copying /etc/sddm.conf.d/kde_settings_conf to /etc/sddm.conf and making sure the correct theme I want is set in the theme section.
If I change the theme again, /etc/sddm.conf is empty and I am back to that lightdm look.
However, the kicker is, it is not consistent. On my laptop, /etc/sddm.conf is always empty and changing the theme works as expected.
Both systems are running the same up to date OpenSUSE Tumbleweed as os and configured identically, with the exception of some persistent shares on the desktop.
My conclusion is that it has to be a local problem, but the question remains where to look and how to fix?
Any hints as to where to investigate?