Done a bit of research:
, our main forum page, the error does not occur when you log in.
-Neither does it if you log in while being on f1technical main page.
The error pop up when:
-You log in on any subforum
-You log on either the "unanswered topics" page or the "active topics" page
-You log on any topic.
It has to be stressed that even though it returns an error, you are actually logged in and from there you can navigate without any problems. Until it's resolved, it will be a minor inconvience. I also think this due the forum upgrade.