There are some settings in the vbulletin software regarding redirects -
It appears to me that the base forum url , www.bladeforums.com/forums redirects to www.bladeforums.com/forums/activity.php - not sure if this was intended or not.
There is also a base url in the admin panel - Admin CP -> Settings -> Options -> Site Name / URL / Contact Details -> Forum URL -
If the forum url there is not correct or the target does not exist, this redirect error can result, although I was never able to get the error to throw in any of my experiments.
I tried with IE and Chrome and several variations of the base url with no luck.
At least you are getting logged in successfully, and this is just a simple page error that you can just navigate away from until our admins have time to check it out.
best
mqqn