I got it working again, not sure exactly what I did to fix it but I booted into safe mode, logged into my account and set it up again (the hi, you've updated windows dialog), then uninstalled UXTheme...

I'm having the same problem, although I can sign in with a different account. I suspect it's either the custom theme I'm using, which required UX Theme Patcher, or the Windows 10 Login Changer I used...