I am currently unable to Quit, but get the following error message instead:
Unable to quit session.
Quitting the session requires that Xfce's session manager (xfce4-session) is running, but it was not detected. Please quit Xfce via another means.
I recently installed a bunch of packages with Package Manager, so I'm suspicious that that is the issue, but unsure of how it happened.
I'm currently just powering off the box until I can fix this.