The ability to login is prevented in IE and FF by a jquery error. It prevents the "log in" button from being clicked after you have entered your account information on the login popup box.
I have not been able to turn this on or off, except by changing browsers.
What I have tried:
Changing Styles - No Effect
Clean Upgrade to 12.1 - No Effect
Enabling & Disabling Countless Options - No Effect
Removing all custom JS from Admin->Look & Feel->Customization - No Effect
Disabled Minify and Optimization Functionality - No Effect
Disabled Cloudflare - No Effect
I have not modified any JS. A clean upgrade confirms this.
I really hope this can be looked at quickly. Users not being able to log in is a serious downer.
See my screenshots for FB Error Reports on the issue.