I've been seeing more of these lately. Enough so that I figured I'd take some time to figure out what the problem is. It turns out that if you're going through an ISA firewall, that you'll see this error when the Web server includes a response that includes an HTTP header with a space or tab at the beginning of the header (this is a Web server problem, not a problem with your ISA firewall).
However, you can change the way the firewall works so that it's more forgiving of this error. Check out the solution over at:
Thomas W Shinder, M.D., MCSE
Sr. Consultant / Technical Writer