Some websites like Facebook and linkedin start off working via HTTP and then switch to HTTPS. If the authentication method is of type "origin" (ip or cookie), and the first request made via HTTPS is the "POST" command used to login, then authentication will fail.

This seems to be unique to Internet Explorer 8 (IE8)

The RFC confirms that it a browser will not automatically follow a redirect to a POST command for security reasons. That is the reason why transparent authentication fails.

The solution is to add a rule in the local policy file (this is not possible via VPM) to bypass authentication for the POST command

define condition HTTPPost
     http.method=(POST)
end condition HTTPPost

<Proxy>
     url.domain="facebook.com" condition=HTTPPost Authenticate(no)
     url.domain="linkedin.com" condition=HTTPPost Authenticate(no)