Go to - Account - History - Manage Login Sessions
Under "current logged in sessions" there should be only 1 line - Current Session. All other lines here are sessions that were not logged out of properly and need to be logged out of manually. Click on the button to the right and they will be moved to the "Prior Sessions" section until you are left with only your current session. DONE!!
Except not quite. Even after executing these steps I could not create a new post. I then guessed the bugged session was the one I was currently in so I used the standard UI "log out" button. After I did that posting appears to have returned to normal.
Apres moi, la deluge