Slightly weird.
When I tried to duplicate it, but with a couple of other subsections and threads open I got this error
Session verification failed. Please try logging out and back in again, and then try again.
When I did log out and back in (refreshing the cookie) it worked correctly.
Looking through the board error log I can't see any errors listed for the last few days from you or anything that looks similar to the error described.
If it happens again can you let me know, including the time?