The remote certificate is invalid according to the validation procedure

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.