I am using FCM HTTP v1 SDK to send Multicast Message as shown below in our C# .NET Application:

        //Send MultiCast Message using SDK
        BatchResponse pushRes = await FirebaseMessaging.GetMessaging(firebaseApp).SendEachForMulticastAsync(message);

Its working perfectly fine on our Dev and QA Environment and notification are being sent successfully to the devices.
But we facing issue on our Production environment with the same service, as its always responding with

FirebaseException.ErrorCode = Unknown

Can someone please help in resolving the same.

NOTE: We were perviously using Legacy FCM API and now we migrating to use FCM HTTP v1 using its SDK