I know there are loads of posts with same issue and most of them were related to proxy and connectivity .
This was case for me as well (few months back). Now the same error is back. But I've confirmed that FW ports and proxy are fine this time around.
server is configured on http port 80
ERROR
Sync failed: UssCommunicationError: 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.~~at System.Web.Services.Protocols.WebClientProtocol.GetWebResponse(WebRequest request). Source: Microsoft.SystemsManagementServer.SoftwareUpdatesManagement.WSyncAction.WSyncAction.SyncWSUS
............
I've checked proxy server connectivity. I'm able browse following site from WSUS server
http://catalog.update.microsoft.com/v7/site/Home.aspx?sku=wsus&version=3.2.7600.226&protocol=1.8
I did telnet proxy server on the particular port (8080) and that is also fine.
I've doubt on certificates, any idea which are the certificates which we need to look? And if certificate is expired then (my guess) we won't be able open the above mentioned windows update catalog site?
Any tips appreciated !
Anoop C Nair (My Blog www.AnoopCNair.com)- Twitter @anoopmannur -FaceBook Forum For SCCM