If you are finding Certificate Warnings within the Application Log on Small Business Server 2008, it may be a sign that the SSL Certificate for Remote Web Workplace (RWW) has Expired, or is about to Expire. This can be checked quite easily within the SBS Console: Network => Connectivity => Certificate => View Certificate Properties. Renew the Certificate if it is appropriate to do so.
These warnings may also be logged if an old SSL Certificate has been superseded, but not removed from the system. This can be checked by running the “Certificates” mmc snap-in on the Server and looking within the Personal => Certificates container.