Hi all,
I have this problem too! Cant recall a specific even that started it, since I've always been able to sign into Youtube.com just fine, via my google account. I tried the above..then tried to sign into Youtube again.....still get the https://accounts.google.com/CookieMismatch page. I have Opera 42, on Windows 7 Home premium. I just opened Internet Explorer...and still signing into my google accnt. on Youtube...working fine.