plasma crashes, losing settings, and disallows logout/reboot

Since my upgrade to kubuntu 20.04, I see my session being messed up completely once per 2 days.
It is always similar, e.g. after a regular logout-login all desktop settings are gone, and I am presented with a native view (attachment follows). my settings are gone, except the icons are up. Background is gone, too. And, my session usually being 'Desktop', it comes back as 'Folder view'.

Also, no way to logout, shutdown, reboot. Only a hard reset is possible with the power switch. All the logout applets (panel, desktop) do simply nothing. Entering a konsole is possible, but anything typed in there starts krunner, and shows consequently up in krunner. Clicking inside the konsole renders the entry character black, but any keyboard input reverts back into krunner.
Yes, this is reproducible. Next time I'll take a video if desired, because it sounds too crazy.
Just now was the fourth time of exactly the same within some 8 or 10 days.
Everything else works perfectly okay and with the expected speed. The panel is always unharmed.

Something very rotten: even the power button on this machine doesn't pop up the shutdown option; so I have to press some 10 seconds to cut the power for a hard shutdown.
Clicking any of the shutdown options show the input (feedback), but nothing happens.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: plasma-desktop 4:5.18.5-0ubuntu0.1
ProcVersionSignature: Ubuntu 5.4.0-56.62-generic 5.4.73
Uname: Linux 5.4.0-56-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.13
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: KDE
Date: Mon Dec 7 18:06:28 2020
InstallationDate: Installed on 2019-02-16 (659 days ago)
InstallationMedia: Kubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
SourcePackage: plasma-desktop
UpgradeStatus: Upgraded to focal on 2020-11-30 (7 days ago)