For people still getting session errors, here's a summary of the steps that worked for me:
(1) Clear the browser cache and cookies in any browser you use to access the form (most importantly, clear the browser cache in the one you're going to log out from).
(2) Close and re-open that browser.
(3) Go back to the forum.
(4) Log out.
(5) Press the login button on the menu bar (next to the Register button, not the spot where you actually put the username and password in),
(6) Uncheck the box that says "Always stay logged in"
(7) Now log in.
That fixed most of the errors for me.
NOTE: You'll likely have to log back in to any other sites you were logged in to before clearing your browser cookies. It will make your browser forget about what it was logged in to before as well as potentially some other preferences for sites that use cookies to store that information about you.
If you only flush the browser cache / cookies, it will fix the problem only temporarily. It will come back shortly. You also have to uncheck "Always stay logged in."
It's almost like the forum is expiring sessions in a different way than before but the browser doesn't know it, so if you leave it logged in all the time, your session expires and gives you errors, even logging out (you can't log out of a session if the session you were logged in with isn't valid anymore
)
Even after the workaround above, there are still some weird errors happening to me. For example, if I leave my browser open and logged in to the forum, whether I'm in the process of composing / editing a post or not, it sometimes starts a chain of errors. The good news is that those errors will clear up if you just refresh the page (hopefully you weren't in the process of posting anything at the time).
Hopefully this helps, everyone.