I know this problem. I encounter the same problem as I convert my 6.5.5 to 7.0
The problem comes from corrupt cookies/leftover old cookies. I clear out all cookies from the domain in my browser by hand (not with the clear cookie function from threads), close the browser, reopen and go to my site.

From this point the problem was gone for me and never come back.

my board: http://www.dragonclan-forum.de
my hobby: http://www.biker-reise.de
Ich kann bei Fragen zu UBBthreads in Deutsch weiterhelfen oder es zumindest versuchen