In the most recent update, a lot of navigation variables got renamed. What we didn't realize was that this change would break all existing hyperlinks - and we've got a lot of them in the forums. Until now, all navigation was done via POST requests, which are by design not stored in urls, thus couldn't be stored persistently, and thus we didn't have to worry about breaking anything. This is no longer the case.
I can come up with two ad-hoc solutions... either a permanent mod_rewrite rule, or an sql query to repair all existing content. Me and Mojko should have this fixed tomorrow. Sorry for the inconvenience.