I've been having this problem as well, then I decided to try some other MS sites, and noticed that I wasn't currently signed in to my passport account, so I signed in. I then tried to go to xbox.com, and it worked no problem. Hope this helps
I have xbox.com save my e-mail address and password. I recently changed my password and I was getting this error until I was able to get to the sign in screen again and change my password. I had to go a round-about way through the Microsoft website to get the page that asks for your e-mail address and password.
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