Bug#987430: upgrade-reports: KDE Plasma without panels and without background after upgrade from Buster to Bullseye

Package: upgrade-reports
Severity: important

My previous release is: buster (fully updated)
I am upgrading to: bullseye
Archive date: Fri Apr 23 14:17:55 UTC 2021
Upgrade date: Fri Apr 23 16:00:00 UTC 2021
uname -a before upgrade: Linux hostname 4.19.0-16-amd64 #1 SMP 
Debian 4.19.181-1 (2021-03-19) x86_64 GNU/Linux
uname -a after upgrade: Linux hostname 5.10.0-6-amd64 #1 SMP 
Debian 5.10.28-1 (2021-04-09) x86_64 GNU/Linux
Method: apt upgrade --without-new-pkgs && apt full-upgrade

I have now upgraded three different machines from (fully updated) 
Buster to Bullseye, and all three times KDE Plasma was not usable 
afterwards.

Symptoms:
- No panels are shown and instead of the desktop/background the 
  screen is black.
- Right-click on the desktop works, but the settings do not allow 
  me to set a background image.
- Pressing the super key opens the launcher menu, next to where 
  the panel should be shown.
- Alt+Tab works, kwin is running normally
- krunner was unusable on two of the machines, but worked on the 
  third.

I think the same problems was described by a reddit user here:
https://www.reddit.com/r/debian/comments/li4vvc/debian_bullseye_has_entered_softfreeze/gnbp9up/?context=3

WORKAROUND:
The problem goes away after reinstalling everything that is 
installed and has "plasma" in the name, but unfortunately I cannot 
say which of those package reinstalls is actually doing the trick:

sudo apt install --reinstall kdeplasma-addons-data 
libkf5plasma5:amd64 libkf5plasmaquick5:amd64 
libplasma-geolocation-interface5 libreoffice-plasma 
plasma-dataengines-addons:amd64 plasma-desktop plasma-desktop-data 
plasma-disks plasma-framework plasma-integration plasma-nm 
plasma-pa plasma-runners-addons:amd64 
plasma-wallpapers-addons:amd64 plasma-widgets-addons 
plasma-workspace plasma-workspace-data


After this and logging out and back in, all of KDE Plasma works as 
normal.