Invalid grant issue with Google OAuth authentication in Qt

[HRZZRH]

I'm developing a Qt application and I want to use Google authentication for it. I created a Google API as explained in the following link: https://blog.qt.io/blog/2017/01/25/connecting-qt-application-google-services-using-oauth-2-0/ but I have a problem with it. It doesn't work in many cases and I get ProtocolInvalidOperationError(302) error for https://accounts.google.com/o/oauth2/token request URL in
QOAuthHttpServerReplyHandler::networkReplyFinished(QNetworkReply *reply)
method of Qt class.

Note that I override QOAuthHttpServerReplyHandler::networkReplyFinished(QNetworkReply *reply) to get this error, because it doesn't emit any signal in this case, and the return value for reply->readAll() is as below:

{
  "error": "invalid_grant",
  "error_description": "Malformed auth code."
}

My Login.cpp code is something as below:

Login::Login() {
    google = new QOAuth2AuthorizationCodeFlow;
    google->setScope("email");
    google->setAuthorizationUrl("https://accounts.google.com/o/oauth2/auth");
    google->setClientIdentifier(Utility::decrypt(encryptedClientId));
    google->setAccessTokenUrl("https://accounts.google.com/o/oauth2/token");
    google->setClientIdentifierSharedKey(Utility::decrypt(encryptedClientSecret));

    connect(google, &QOAuth2AuthorizationCodeFlow::authorizeWithBrowser,
      &QDesktopServices::openUrl);

    connect(google,&QOAuth2AuthorizationCodeFlow::authorizationCallbackReceived,[=](const QVariantMap data){

    QString code(data["code"].toString());
    if(!code2.isEmpty())
    {
        const QUrl redirectUri= "http://localhost:56413/cb";
        QJsonObject postdata;
        postdata.insert("code",code);
        postdata.insert("client_id", Utility::decrypt(encryptedClientId));
        postdata.insert("client_secret", Utility::decrypt(encryptedClientSecret));
        postdata.insert("redirect_uri", redirectUri.toString());
        postdata.insert("grant_type","authorization_code");

       QString serviceURL = "oauth2/v4/token";
       NetworkManager::GetInstance()->Post(postdata,serviceURL,"https://www.googleapis.com/",[=](int statusCode,int resultnumber, QJsonObject obj){
		   if (statusCode >= 200 &&   statusCode < 300) {
			   // it's ok, do nothing
		   }
		   else {
               //show error
                   }
           });  
        }
    });
}

void Login::googleLoginButtonPressed() {
    int googlePort = 56413;
    if(replyHandler == nullptr)
	replyHandler = new QOAuthHttpServerReplyHandlerArio(googlePort, this);
    google->setReplyHandler(replyHandler);

    QObject::connect(replyHandler, &QOAuthHttpServerReplyHandler::tokensReceived, [=](const QVariantMap &map) {
	googleToken = map["id_token"].toString();
	connect(google, &QOAuth2AuthorizationCodeFlow::granted, [=]() {
		auto reply = google->get(QUrl("https://www.googleapis.com/plus/v1/people/me"));
		connect_reply = connect(reply, &QNetworkReply::finished, [=]() {
			int statusCode = reply->attribute(QNetworkRequest::HttpStatusCodeAttribute).toInt();
			if (statusCode >= 200 &&
				statusCode < 300)
			{
				//NOW register or login the user with email

				QJsonDocument jsonResponse = QJsonDocument::fromJson(reply->readAll().data());
				email = jsonResponse.object().value("emails").toArray()[0].toObject().value("value").toString();
				reply->deleteLater();
			}
			else {
				//error
			}
		});
	});
    });

    google->grant(); 
}

Also my Qt version is 5.9.1.
what's the problem?

Thanks for your help.