I just brought up Yahoo home page and signed in to my Yahoo account with no delay in IE-11. However, Microsoft's EMET security tool popped up a warning of invalid certificate when doing so. No other browser or AV warnings appeared.
Since EMET detects SSL/TLS certificate trust pinning, an inconsistency which could indicate a fraudulent certificate possibly by a Man in the Middle(MIM) attempt, I would find this suspicious? Warrants a closer look, but could also be a configuration issue with Yahoo servers.
I get the 'invalid certificate' window at least twice a week. I just click it away and keep on going.