Same Here! After flashing another MIUI Rom it won't let me log in into my mi account for the same reason.
But I have also noticed that I am not able to log in into my account from my computer via the browser. Mi Cloud should be accessible via https:// i.mi. com. But - surprise, surprise - it fails also due to a connection error (http 408?).
So I guess, we just have to wait for a fix on Xiaomi's side? Can anybody confirm my observations?