SUMMARY
SDDM / Login works OK when system first boots up. If I log out, I can't log in again. Much discussion in Gentoo forums. See https://forums.gentoo.org/viewtopic-t-1136583.html and also https://forums.gentoo.org/viewtopic-t-1135770.html.

STEPS TO REPRODUCE
1. Boot the system and log in.
2. Log out.
3. Segmentation fault in kwin; can't log in again.

OBSERVED RESULT
I can't log into my Plasma session unless I restart sddm.

EXPECTED RESULT
I should be able to log out, then log back in again. It worked until a couple of days ago. I suspect "qtcore" and/or "qtgui", which were recently upgraded in Gentoo.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.20.5
KDE Frameworks Version: 5.80.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
I'm reporting this as a bug in Plasma, with the X11 windowing system. But it also appears to be killing the Wayland windiwing system. I usually don't use Wayland, but it used to work, and now it doesn't. I can't even get a decent error message out of Wayland. I also have XFCE configured ... amaxingly enough, I was able to get some screenshots of Drkonqui after kwin had crashed by logging in to XFCE and using those tools. Screenshots are attached.