To resolve the issue I encountered with my iOS app in Firebase, the most effective solution was to delete the app from Firebase and re-register it. Additionally, I deactivated and then reactivated the authentication features. Unfortunately, I found that the support from Google was not helpful, despite having paid for a support plan. If you're facing a similar problem, I recommend skipping the time-consuming attempts at getting help from Google and simply follow these steps to resolve the error quickly.