The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel

I'm using SQL Server 2005 (SP1) Reporting Services on Windows Server 2003 SP1. I access the report server from Internet using SSL, but the server is not exposed directly to the internet. It is behind a firewall, where the required ssl port is setup and is working fine.

In my Reporting Server, the /ReportServer folder works perfect and I can navigate and view all reports. It is the /Reports folder the one that does not work fine. I can enter the /Reports folder, and can list all the reports. But, when I select one of the reports, and it starts rendering, I get the error message:

"The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel. The remote certificate is invalid according to the validation procedure."

How can I fix this?

Thanks,

Julio