tl;dr You have to press enter on the text boxes for the name and password in order to type in it, it will light up green when you can type and be orange when you can't.
I figured it out! It's not a bug, you just have to press enter on the text box, type in your name, then press enter again to...
I'm on Windows 8.1 and I have the IME off as well but I'm also having the same problem. When I go to sign in the keys on my keyboard don't type at all but I can move the cursor up and down with the arrow keys. Is that the same with you?