None of the screen lock tools ran successfully

If you have changed your screen locker you may get the error None of the screen lock tools ran successfully when returning to your computer after a period of inactivity. This is caused by some of the xfce configuration files under your home directory being messed up.

To cure the problem:

1. Restart the computer,
2. Do not log in to the GUI, instead log in to a tty session.
3. cd ~/.cache/sessions && rm -rf *
4. cd ~/.config/xfce4/xfconf/xfce-perchannel-xml && rm xfce4-power-manager.xml xfce4-session.xml
5. Log in to the GUI

This should cure the problem, but you will have lost any saved sessions and will have to re-save your screen layout, if you so desire.