I did a great deal of testing to figure out the cause, and on my site, when Simple-Press is enabled (even if every other plugin is disabled and the theme is set to twentytwelve), any Log Out link results in a WordPress Failure Notice. The page that says:
You are attempting to log out of HerbMentor
Do you really want to log out?
Clicking the link results in a Server Error. If the site is visited again, the user is logged out, but this is a HUGE usability bug.
Simple-Press isn’t set up to allow people to register, and the Login/Logout redirect settings are blank.
PLEASE HELP!