I spent a day trying to fix this and found out it’s not really a problem. When you create a custom extension for authentication using the TokenIssuanceStart event and connect it to your app, you might see this error when first time you run the user flow. But if you wait a few minutes and try again, the error goes away. So it’s not a real issue — Entra ID just needs some time to update the changes after the first time execution of TokenIssuanceStart event.