Right password.... That worries me as you should be either using oauth authentication or a mailkey. Neither of those are your password.

So what is the error message exactly (verbatim, not some translation or abbreviation)

What form of Authentication are you using in account settings?

The error is "Unable to log in at server. Probably wrong configuration, username or password." I've tried both Normal Password and OAuth. Apparently I'm unable to create a mail key with this account at AT&T. I can't do anything with the account, like forward the email, or create rules, nothing. Thanks for responding, I appreciate it.

I don't know whether the steps below are accurate, but Thunderbird can use an app password and you would need to change the Thunderbird authentication to "normal password".

For ameritech.net/att.net (AT&T) accounts

• You will need an app password if you are using an older, less secure third-party email client.
• Log in to your AT&T Mail account via a web browser.
• Navigate to your account settings to find the option to generate an app password.
• If you are using the official AT&T or Yahoo apps, you do not need an app password; you can use your main account password

Yahoo has backed out their breaking change.

Yahoo has backed out their breaking change.

I don't know what this means.

Yahoo several days ago made a change to their systems.

Users reported problems

Yahoo has now removed the changes that they made several days ago.