When Yahoo Publisher Network publishers try going to the YPN Portal, they are now presented with a security prompt. If you look closely at that error, it says that their secure certificate has expired.
It expired a day ago. I guess Yahoo is trying to cut costs wherever they can, including security. Of course I am kidding. Often companies forget to renew their security certificates. I am sure it will be resolved shortly and I am sure the site is still secure, it just doesn't have the Equifax seal of approval.
Forum discussion at WebmasterWorld.