Getting TV Provider "Sign In Error" for AT&T TV login

I can't get the single "TV Provider" sign in to work for AT&T TV. I have verified that I have the correct password and login by successfully signing directly into the web site browser version of AT&T TV.

But each time I try to connect my AT&T TV account via the iOS or Apple TV settings for "TV Provider", it says "Verifying" for about a minute and then eventually gives me a pop up that says "Sign In Error". There's definitely some back end screwy stuff happening, because if I put in a wrong password on purpose, it immediately says "Sign In Error: That user ID and password don't match." But if I put in the correct credentials, it will hang on "Verifying" for a long time and eventually time out with the message "Sign In Error".

Would love to use the single sign on feature. And have had success in the past. But currently it doesn't seem to work with AT&T TV.