For your reference, here is the actual configuration we have registered in Apple Developer: Service ID (client_id): com.exrm.service.local Return URL: https://611f-121-160-153-88.ngrok-free.app/login/oauth2/code/apple Tested Authorization URL: https://appleid.apple.com/auth/authorize?response_mode=form_post&response_type=code&client_id=com.exrm.service.local&scope=name%20email&state=zHWg8cYzYe1RwahcS-1rghLbYFjZngAtFwMqZMNGeFc%3D&redirect_uri=https://611f-121-160-153-88.ngrok-free.app/login/oauth2/code/apple We are seeing the same invalid_client error when using this setup. Would you please confirm if everything looks correctly configured, or if anything additional needs to be done on our side?