An authentication error will be displayed if you do not perform two-factor authentication using authentication code within the set time limit after authenticating using your email address and password.
If you enter the authentication code after this time limit, you will be sent back to the sign in screen and the following error message will be displayed.
Enter your email address and password to authenticate again, and then try performing two-factor authentication quickly after doing so.
If multiple "Mirai Translator" accounts are displayed in the authentication application, try deleting the old accounts but keep the most recent one.
If the problem is still not resolved after trying the above, see here.