INVALID_APP_CREDENTIAL for GoogleIdentityToolkit API

[Prashant Kumar Mishra]

I am using GoogleIdentityTookit for authenticating phone number by sending OTP for my web application. I have followed this documentation for implementation. Initially everything was working fine but from few days back it started giving below error.

{ "error": { "code": 400, "message": "INVALID_APP_CREDENTIAL", "errors": [ { "message": "INVALID_APP_CREDENTIAL", "domain": "global", "reason": "invalid" } ] } }

And on the UI I am getting 
Error: The phone verification request contains an invalid application verifier. The reCAPTCHA token response is either invalid or expired.

here is my code snippet

<script src="https://www.gstatic.com/firebasejs/6.3.3/firebase-app.js"></script>
<script src="https://www.gstatic.com/firebasejs/6.3.3/firebase-auth.js"></script>

<script type="text/javascript">
    var firebaseConfig = {
      apiKey: "AIz************************************"
    };

   firebase.initializeApp(firebaseConfig);
   window.recaptchaVerifier = new firebase.auth.RecaptchaVerifier("recaptcha-container", {
  size: "invisible"
});

//This function is called on a button click

function sendOTP() {
  firebase
  .auth()
  .signInWithPhoneNumber(phoneNo, appVerifier)
  .then(function(confirmationResult) {
    window.confirmationResult = confirmationResult;
    if(modal.style.display !== 'block') {
      openOTPModal(phoneNo);
    }
  })
  .catch(function(error) {
    phoneNumberErrorMessage(error);
  });
}

</script>

Note:- This is not failing for every request, but in most of the requests are failing.

Thanks in advance for any help!