If you encounter a 500 internal server error while attempting to log in to your TFX account using your Apple ID, it may be because you began the TFX sign-up process with your Apple ID but did not complete it. This may be due to Apple's security policy, which prohibits the transfer of your email data more than once. To resolve the issue, you should revoke TFX's authorizations on your Apple ID page and sign up again.
To do it:
Sign in to your Apple ID account at https://appleid.apple.com/ and navigate to Sign-In and Security.
Click Sign in with Apple.
Click Taxes for Expats in Sign in with Apple pop-up.
Click the Stop using Sign in with Apple button.
Confirm this action by clicking the Stop using button.