Technically, SSL is still working fine. The 404 page-not-found result is delivered over the secure channel :) I know nothing about the Microsoft Bing servers,

Technically, SSL is still working fine. The 404 page-not-found result is delivered over the secure channel :) I know nothing about the Microsoft Bing servers, but my stab-in-the-dark guess would be that somebody changed something on the load-balancers that handle SSL termination, and now they're proxying to the wrong pool of back-end servers.