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:
Be sure to use your own username and groupname in these commands.