After upgrading from Kubuntu 20.04 to Kubuntu 20.10, I could not login as it just showed a black screen after boot. So I switched to command line and reinstalled Kubuntu desktop with sudo apt-get install --reinstall kubuntu-desktop kde-*. After that, i was able to login and can use all applications but the icons and background of input boxes of system apps have turned black as shown below
Notice the black squares at the bottom right in the system tray, they used to be icons for Volume, Network, Bluetooth, etc. I can however click them and they behave as they used to. Also the desktop apps' icons are black.
Notice the input fields at the right of Layout, Wallpaer type and also the button backgrounds of the window.
After looking at some logs I noticed that Akonadi server was not starting up because of an unknown MySQL error and installed MariaDb as i read somewhere that it might help. Now I can start Akonadi server with akonadictl manually but still the same issue. I restarted plasma with kquitapp5 plasmashell && kstart5 plasmashell but still same issue. Also Akonadi does not start automatically when session begins.
Best Answer
Found the solution here, as per the link in the link:
so make a:
sudo apt install libqt5quick5it worked for me on debian