Is Yahoo mail the only site that gives you a certificate error? if this is occurring on many different sites, we'll suggest some diagnostics to try to run that down. Does it make any difference if you access mail through this general domain: mail.yahoo.com instead of your regular approach? One thing that seems to be affecting a lot of Windows 10 users is that parental controls are being applied to their accounts, which causes seemingly random blockages. Could you check Microsoft Family Safety and either disable it or take your account out of the family: http://windows.microsoft.com/en-us/windows-10/turn-off-microsoft-family-settings