Desktop settings (background and applets) disappear in Kubuntu 20.1

Solution 1:

UPDATE: Well I couldn't find help anywhere, I tried some solutions that I found for this problem but they did not worked for me, maybe because were for older versions. So I reinstalled Kubuntu and repeated everything I did checking on every step (updating the system, installing drivers, installing software, changing some devices setup, etc) and found the cause that also works for Kubuntu 20.04, which I also tested and got the same problem. So I will post it here in case someone else has the same issue.

The problem was caused by a global theme. After downloading and installing the theme "sweet KDE" my desktop turned black and every setting on the desktop stop being saved. I also installed and applied other 4 themes to check if they triggered the issue, and they didn't.

The "sweet KDE" theme also caused another issue in KUBUNTU 20.04 (NOT in 20.10): after installing the theme I could not restart, poweroff, or close session because the "KDE greeting"(I don't remember the full name of the app) froze leaving a blur screen with no options, so the only way to poweroff or restart was using the terminal.

THE SOLUTION: uninstall the theme. That's it. After uninstalling the theme all the problems went away and everything is working as it should again :)

The only annoying thing is that although I uninstalled the theme I still see it in the list of installed themes.

I hope this helps anyone that crosses with this kind of issue.