Here another perhaps helpful partial remedy during bug-season:
It helps to log out and/or close the session and start a new session.
If I've seen "all cookies cleared" the remedies I mentioned work i.e. then
(1) "What's new" and "New Posts" in the topmenus then work if not logged in
(2) Posting works if one logs in via the thread
But as said before: Install the previous version of the forum-software, or at
least reinstall the version that's now buggedly active seems the best option
to try out to get rid of this set of bugs.
The forum-software is by far the likeliest candidate for the cause of this.