I still say try the ‘configuration related’ steps:
(I will augment them this time so that everything is backed up)
cd ~/.config
for j in plasma*; do mv -- "$j" "${j%}.bak"; done
mv Trolltech.conf Trolltech.conf.bak
kbuildsycoca5 --noincremental
cd ~
Reboot.
See if its ‘fixed’ … if so … start moving your configurations back one-by-one … find the culprit.