I understand that you experience user signup issue after moving your App to a different domain.
Here are two FAQs related to updating domain in Auth0. By searching with custom domain #faq in our Community, you may find more FAQs.
If the error persists, I can dig further into this if you DM me additional details such as the name of your tenant and the App, if the error happens when signup in the UI or/and with management API, and the details of the exception/error.