I have the same problem, I opened even a thread today earlier.
But clearing everything, doesn't always solve the problem for me.
Sometimes it does, but even in this case after 2-3 hours it will happen again the same problem.
But mostly of the times even that I clear everything, cookies cache, I still cant access the forum.
I have to access it through anonymouse. Like this, it is annoying and slow as hell, but what to do :S