Doubleclick.net is an add site- this site uses them to display some of the adverts.
Are you seeing "Not authorised to view this page" in areas of a page where adverts would normally appear?
If you are, then it's probably down to the functioning of your firewall/add blocker.
It's showing the message because it's blocking the content from Doubleclick.net.
What happens when you click Back (IE6)- nothing?