FireBaseAuth not working in TIER-App

Dear all,
when TIER App tries to perform a SMS-auth via FireBaseAuth, the microG overlayer appears but nothing happens. Below I attached a logcat. I stripped off the key part of the URL https://www.googleapis.com/identitytoolkit/v3/relyingparty/getProjectConfig?key=[replaced] because I'm not sure whether it contains sensitive info. I'm running microG 0.2.13.203915, Android 10, device is lavender.

11-25 22:24:41.221 D/GmsFirebaseAuth( 4035): onBind: Intent { act=com.google.firebase.auth.api.gms.service.START pkg=com.google.android.gms }
11-25 22:25:00.825 D/GmsFirebaseAuth( 4035): sendVerificationCode
11-25 22:25:01.168 E/Volley ( 4035): [333] BasicNetwork.performRequest: Unexpected response code 403 for https://www.googleapis.com/identitytoolkit/v3/relyingparty/getProjectConfig?key=[replaced]
11-25 22:25:01.363 W/WindowManager( 1610): Unable to start animation, surface is null or no children.
11-25 22:25:01.470 E/Volley ( 4035): [333] BasicNetwork.performRequest: Unexpected response code 403 for https://www.googleapis.com/identitytoolkit/v3/relyingparty/getProjectConfig?key=[replaced]
11-25 22:25:01.471 W/GmsFirebaseAuth( 4035):
11-25 22:25:01.471 W/GmsFirebaseAuth( 4035): java.lang.RuntimeException: com.android.volley.AuthFailureError
11-25 22:25:01.471 W/GmsFirebaseAuth( 4035): at org.microg.gms.firebase.auth.IdentityToolkitClient$getProjectConfig$2$2.onErrorResponse(IdentityToolkitClient.kt:58)
11-25 22:25:01.471 W/GmsFirebaseAuth( 4035): at com.android.volley.Request.deliverError(Request.java:617)
11-25 22:25:01.471 W/GmsFirebaseAuth( 4035): at com.android.volley.ExecutorDelivery$ResponseDeliveryRunnable.run(ExecutorDelivery.java:104)
11-25 22:25:01.471 W/GmsFirebaseAuth( 4035): at android.os.Handler.handleCallback(Handler.java:883)
11-25 22:25:01.471 W/GmsFirebaseAuth( 4035): at android.os.Handler.dispatchMessage(Handler.java:100)
11-25 22:25:01.471 W/GmsFirebaseAuth( 4035): at android.os.Looper.loop(Looper.java:214)
11-25 22:25:01.471 W/GmsFirebaseAuth( 4035): at android.app.ActivityThread.main(ActivityThread.java:7356)
11-25 22:25:01.471 W/GmsFirebaseAuth( 4035): at java.lang.reflect.Method.invoke(Native Method)
11-25 22:25:01.471 W/GmsFirebaseAuth( 4035): at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
11-25 22:25:01.471 W/GmsFirebaseAuth( 4035): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:940)
11-25 22:25:01.471 W/GmsFirebaseAuth( 4035): Caused by: com.android.volley.AuthFailureError
11-25 22:25:01.471 W/GmsFirebaseAuth( 4035): at com.android.volley.toolbox.BasicNetwork.performRequest(BasicNetwork.java:195)
11-25 22:25:01.471 W/GmsFirebaseAuth( 4035): at com.android.volley.NetworkDispatcher.processRequest(NetworkDispatcher.java:131)
11-25 22:25:01.471 W/GmsFirebaseAuth( 4035): at com.android.volley.NetworkDispatcher.processRequest(NetworkDispatcher.java:111)
11-25 22:25:01.471 W/GmsFirebaseAuth( 4035): at com.android.volley.NetworkDispatcher.run(NetworkDispatcher.java:90)

Start with the FirebaseAuth flow and the request shown at IdentityToolkitClient.kt:58, then reproduce SMS authentication in TIER-App using the reported Android 10 and microG 0.2.13.203915 setup. Use logcat to investigate the 403 response from getProjectConfig; done means the request no longer fails and SMS authentication proceeds.