After update, some of my Plasma activities settings was lost. I lost widgets on desktop: notes, icons. Also, settings of desktop was lost (wallpaper for example). Only panels for Plasma was preserved.
I do not ask how to being it back, because I change and log out after seen settings was lost. I only asks some else have the same problem.
Have you used sudo or su or su - lately? These might cause loss of ownership of some of your files and/or directories, making them unwritable by you. To correct this problem you could login as root after Ctrl-Alt-F3 and then:
chown -R username:groupname /home/username
Something else that could possibly help is while Plasma is not running, run:
rm /home/username/.cache/*
Be sure to use your own username and groupname in these commands.