Normally I try not to post in threads that are more than 90 days old, but in this case I must, because this has happened to me twice this month.
The first time around I had to restart my computer, run my anti-virus program and clear my entire browser cache/history before I was able to log into the forums.
The second time around I just closed my browser, ran my anti-virus and was able to login again, however, now, every time I try to reload the forums, I'm forced to login again, even though I have the "Keep me logged in forever" box checked.
I don't know if it'll help, but every time I got this error, I would get this string of letters and numbers on the error page...
4cb3-fc9b-b40c-8ddc
I honestly don't know jack about these sort of issues, so I hope it's nothing serious.