How to restart KDE 5 interface?
Solution 1:
In a terminal (ctrl+alt+Fx) (if KDE is stuck use tty) type:
$ kquitapp5 plasmashell
and, eventually,
$ plasmashell
to restart, or
$ kstart5 plasmashell
if you want to be able to logout of the tty.
Solution 2:
For >=kde5.10 (Kubuntu20.04; plasmashell 5.18.5):
Open a console CtrlAlt+T
login and type:
kquitapp5 plasmashell
kstart5 plasmashell&