You may see this error in your Windows Event Log:
System.Net.WebException: The underlying connection was closed: Could not
establish trust relationship for the SSL/TLS secure channel. --->
System.Security.Authentication.AuthenticationException: The remote
certificate is invalid according to the validation procedure.
This is generally caused by some problem with the SSL certificates installed on your server.
There is a hotfix for this on Windows Server 2003 and if that doesn't work, Microsoft has a good guide for trying to track this down on the MSDN blog.