Fresh user profile

Log out, open TTY 1 (CTRL+ALT+F1), log in with your p-number.

If you want to save the dot files:

mkdir ~/backup_dot_files && mv ~/.* ~/backup_dot_files/

NOTE! this will give you the following error message that you can ignore:
mv: cannot move '/home/p123456/.' to '/home/p123456/backup_dot_files/.': device or resource busy
mv: cannot move '/home/p123456/..' to '/home/p123456/backup_dot_files/..': device or resource busy

If you have no settings to save, you can do this:

rm -rf ~/.* 

NOTE! this will also give you an error message that you can ignore:
rm: refusing to move '.' or '..' directory: skipping '/home/p123456/.'
rm: refusing to move '.' or '..' directory: skipping '/home/p123456/..'

Switch back to the graphical user interface (GUI) (CTRL+ALT+F7) and log in. If everything works again, don't forget to logout of TTY1 by giving the command:

exit

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The following directories containing most of the application settings you did yourself. If you want to keep your settings make a copy of these directories:

• .local/
• .config/

After cleaning your profile and logging in works again, you can copy back these two directories the same way you removed them; Log out, open TTY 1, log in with your p-number, copy the directories and login to the GUI.
If the trouble returns, the problem is in one of those directories. Clean your home dir again and now copy back your settings one at a time to find the culprit.

I also made a, in no way complete, list of directories that might be in your home dir which sometimes are the cause of trouble and you can delete without thinking about it twice:

• .dbus/
• .gnome/
• .gnome2/
• .gnome2_private/
• .qt/
• .nv/
• .nvidia-settings-rc
• .Xauthority
• .gconf/
• .cache/
• .kde/
• .compiz/