I would advise to log out of Plasma completely, and when you are seeing the login screen reappear, press Ctrl+Alt+F2 to switch over to a tty. Log in at the tty with your normal user name and password. Then issue the following command… 
rm -rf ~/.cache/*
Press Ctrl+D to log out of the tty, and Alt+F1 to return to the graphical login screen. Log back in and see whether it makes a difference.