Missing icons on the login screen and everywhere else after upgrade to Kubuntu 21.04? You have come to the right place!
Easy fix: sudo apt install libqt5quick5
I did not have any problems on my main machine, but my old machine which had not been updated for more than a year did not swallow the upgrade easily.
You might want to look here for some other fixes and tips & tricks: https://zren.github.io/kde/
Some other tweaks
kwriteconfig5 --file startkderc --group General --key systemdBoot true
/etc/systemd/logind.conf > KillUserProcesses=yes
Taskbar Task Manager Missing Plasmashell
Remember to backup first. Reset the file plasma-org.kde.plasma.desktop-appletsrc: 'rm ~/home/USER/.config/plasma-org.kde.plasma.desktop-appletsrc'