Error performing web request. The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.

I have a site workflow, which makes call to SAP and returns data. When I run the workflow manually it works fine but When I schedule the same it throws the following error "Error performing web request. The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel."

I Updated all the certificates under "Manage Trust". I wonder whey it throws this error when I schedule the Workflow. 

Any help would be appreciated. Thanks