Click on the lock icon and "View Certificate". It should show you the yahoo cert and any in the chain. You can click on each in the chain. If any don't say "Valid Cert" there there is some issue with that cert. Post a screenshot of any cert with warnings.
Thanks - certificate was valid, but in view of all the yahoo problems I have switched everything to microsoft. Yahoo calendar no longer offered, and the email quit working on their site when I opened windows mail accounts. Oh, well.