I am getting a 401 response when hitting the Power BI login page! https://app.powerbi.com/?noSignUpCheck=1
The trick is actually getting to the cookies that got themselves into a mess that prevented logging in.
I'd normally have the devtools panel up and you just couldn't see the cookies or anything there where you'd expect them.
But if you click on the little SSL lock/Info button...
(In this case I've already resolved the problem but what it did show was 10 cookies associated with the failed app.powerbi.com site)
You can then click the Cookies item and it takes you to the Chrome settings page for app.powerbi.com
And from there you can actually clear the broken cookies and refresh the page (app.powerbi.com) to log in once again!