Since 8.1 update to Surface, I can't access office 365 portal, nor yahoo, "This certificate cannot be verified up to a trusted certification authority"

I have spent at least 10 hours doing every imaginable fix recommended, and none of them have the least bit of effect on this problem.  Unbelievably frustrating, "certificate errors" and can't get to the Office 365 portal, and can't set up Outlook.  MS didn't put an automatic restore point on my computer before doing the 8.1 upgrade, so I'm screwed there. Dead in the water, this Surface tablet is a doorstop at this point. Swear word.