[firebase_ui_auth][example] - Verify your email responds with 17999 error code

[FuadEfendi]

Hello,

I am trying to reproduce provided Register User example with my own Cloud Firestore instance and I have error 17999 (which can only be seen if you debug code); FIRAuthErrorCodeInternalError

It happens when I input Email, and password (twice), on the "Verify your email" step.
So far, the only difference with the example code is Cloud Firestore (my app; fails with 17999) vs. Realtime Database (official example, works just fine).

What could be wrong, and how to troubleshoot it? I enable IDM & audit but cannot find anything.

Also, is it possible to browse 'https://flutterfire-e2e-tests-default-rtdb.europe-west1.firebasedatabase.app' and compare with my settings?

Thanks,