Desktop Icons disappeared, icons in taskbar not working, slow log in

[cscs]

I was pretty much going to suggest what @jcorporon sorta-linked …

Continuing the discussion from KDE Plasma Completely dysfunctional:

@cscs

cd ~/.config
for j in plasma*; do mv -- "$j" "${j%}.bak"; done
rm ~/.config/Trolltech.conf
kbuildsycoca5 --noincremental

And maybe the cache too?

rm -rf ~/.cache/*

Though of course you could backup the Trolltech file and/or the .cache folder instead of just deleting them.

And do note that it will remove your desktop configurations - you will have to set things again.
(or move back the files)