Its not a vbPortal issue, could be server related.
Reason being if you search vb forums you'll find other have had the same problem but not been using vbportal, in fact one chap had a basic install of vb only. (see
http://www.vbulletin.com/forum/showt...oved%26quot%3B )
Have you changed anything on the server settings ?
One other thing to check, as it was in reply to a similar post on vb.com, and the reply was to: AdminCP -> vBulletin Options -> Cookie and HTTP Header Options -> Remove Redirection Message Pages