If i answer in bottom "quick reply" - it's ok, but if i click any link "quick reply" in menu on page - it's not working write: - The form you have submitted is no longer valid.
Make sure to update all your templates or at least showflat.tmpl and showthreaded.tmpl.. For some reason these templates along other were not detected as changed during the upgrade process.
I know what is the problem. It would be tricky to fix. They strore the form key and form key hash in the session, based on bunch of fields and the parent ID of the last post (silly). Therefore, the form key hash is going to be valid only for the last post in flat mode.