An unusual occurrence but mystifying. Can't replicate the described situation, which adds to the mystery.
Using MATE on RHEL 7. Needed to log out of one profile in order to log in using another profile. Clicked on the "Logout" button. The logout process starts: all controls and desktop items / icons disappear, all I see is the desktop background image.
Pressing CTL-ALT-F4 brings up a tty screen. I login at that point, "su -" up, and enter "loginctl" command. From the results, I see the session number of the process causing the problem. Enter the "loginctl kill-session [session number]" and the problem goes away.
Any ideas of why the problem occurred in the first place?