It's working fine for me. I access
https://bitmain.com/ and Antpool just fine and both redirect me to the login page. It's unlikely since it seems like you know what you're doing but have you double checked the site address?
It doens't work for me either on the passport site (the login site is passport.bitmain.com) when I try to access it from hashnest.
I can log in but can't get past the 2FA sms/call.