Firebase REST Auth on Android returns 500 - firebase

Question

Up until last week, the only resonable path available (for a lot of reasons) to use Firebase in a Xamarin.Android app required using Firebase.Xamarin. However, this appears to have stopped working as well.
Specifically, I had been able to successfully make the following call in my Android and iOS apps and get successful responses:
var contentString = $"grant_type=refresh_token&code=string&refresh_token={auth.RefreshToken}";
var requestContent = new StringContent(contentString, Encoding.UTF8, "application/x-www-form-urlencoded");
var uri = new Uri(string.Format("https://securetoken.googleapis.com/v1/token?key={0}", this.authConfig.ApiKey));
var response = await client.PostAsync(uri, requestContent).ConfigureAwait(false);
var responseData = await response.Content.ReadAsStringAsync().ConfigureAwait(false);
As of yesterday, I started getting the following responseData from the above code:
iOS:
{
"error": {
"code": 403,
"message": "Requests from this ios client application \u003cempty\u003e are blocked.",
"status": "PERMISSION_DENIED",
"details": [
{
"#type": "type.googleapis.com/google.rpc.Help",
"links": [
{
"description": "Google developer console API key",
"url": "https://console.developers.google.com/project/<my_apps_project_id>/apiui/credential"
}
]
}
]
}
}
Android:
{
"error": {
"errors": [
{
"domain": "usageLimits",
"reason": "ipRefererBlocked",
"message": "The request did not specify any Android package name or signing-certificate fingerprint. Please ensure that the client is sending them or use the API Console to update your key restrictions.",
"extendedHelp": "https://console.developers.google.com/apis/credentials?project=<my_apps_project_id>"
}
],
"code": 403,
"message": "The request did not specify any Android package name or signing- certificate fingerprint. Please ensure that the client is sending them or use the API Console to update your key restrictions."
}
}
After some research, I found this Google Cloud Endpoints Google Groups thread that said to be sure the following are in the http request's headers:
Header x-android-package
Label servicecontrol.googleapis.com/android_cert_fingerprint
Header x-android-cert
Label servicecontrol.googleapis.com/android_package_name
Header x-ios-bundle-identifier
Label servicecontrol.googleapis.com/ios_bundle_id
I added client.DefaultRequestHeaders.Add("X-Ios-Bundle-Identifier", <my_apps_bundle_id>); to my code before the client.PostAsync and it works like a charm.
Moving on to Android, I added "x-android-cert" and x-android-package to the request headers:
client.DefaultRequestHeaders.Add("x-android-cert", <my_apps_SHA1>);
client.DefaultRequestHeaders.Add("x-android-package", <my_apps_package_id>);
prior to my client.PostAsync call. However, now I get the following responseData:
{
"error": {
"errors": [
{
"domain": "global",
"reason": "internalError",
"message": "Internal Error"
}
],
"code": 500,
"message": "Internal Error"
}
}
A moment of venting: For me, this is almost the last straw when it comes to Firebase. It has been over a month of overcoming one undocumented and half-baked API/SDK after another. It seems as if NOTHING works as advertised (including - off topic - getting Firebase working with an Angular4 project). Thank you for letting me share my frustration. Had I known how painful this was going to be, I would have setup my own MBaaS.
Before I give up on Firebase, I wanted to ask for help with this issue. Any insights and assistance would be greatly appreciated.
EDIT 1:*
I get similar results with the following changes:
var requestContent = new StringContent(postContent, Encoding.UTF8, "application/json");
var uri = new Uri(string.Format("https://www.googleapis.com/identitytoolkit/v3/relyingparty/verifyAssertion?key={0}", this.authConfig.ApiKey));