Connection closed while trying to POST

I am new at QT and I am trying to establish REST service (GET and POST) for my application. Meanwhile GET demand works fine, I can not execute POST demand (as result I got from server reply: Connection closed error). Do you have any suggestions what is wrong?

Constructor:

{
    net_acc_manager = new QNetworkAccessManager(this);
    connect(net_acc_manager, &QNetworkAccessManager::finished,this, &MyApp::httpFinished);
}

POST, response I get: Failed, error code #2. Server error explanation: Connection closed

void MyApp::Post_demand()
{
    QUrl url = QUrl("http://192.168.2.1/od/6040/00");
    QNetworkRequest request(url);
    request.setHeader(QNetworkRequest::ContentTypeHeader, "application/x-www-form-urlencoded");
    QByteArray postdata="0000";
    net_acc_manager->post(request,postdata);
    qCDebug(log_hal) << Q_FUNC_INFO << "Request POST posted" << full_url.toString();
}

GET, that part works fine, output: Http post finished successfully.

void MyApp::Get_demand()
{
    QUrl url = QUrl("http://192.168.2.1/od/6040/00");
    reply=net_acc_manager->get(QNetworkRequest(QUrl("http://192.168.2.1/od/6040/00")));
    qCDebug(log_hal) << Q_FUNC_INFO << "Request GET posted" << full_url.toString();
}

HttpFinished function

void MyApp::httpFinished(QNetworkReply *n_reply)
{

    qCDebug(log_hal) << Q_FUNC_INFO << "Http request finished. Status code:" << n_reply->attribute(QNetworkRequest::HttpStatusCodeAttribute).toInt();

    if (n_reply->error()) {
        qCCritical(log_hal) << Q_FUNC_INFO << "Status code:" << n_reply->attribute(QNetworkRequest::HttpStatusCodeAttribute).toInt();
        qCCritical(log_hal) << Q_FUNC_INFO <<
                       QString("Failed, error code: #%1; Server error explanation: %2")
                       .arg(QString::number(n_reply->error())
                            , n_reply->errorString().replace(QRegExp("\%20"), " ")).toLatin1().data();

        n_reply->deleteLater();
        n_reply = Q_NULLPTR;
        emit failed();
        return;
    }

    qCDebug(log_hal) << QString("Http post finished successfully. Reply Text: %1\nElapsed time: %2 ms")
                         .arg(QString(n_reply->readAll()), QString::number(timer->elapsed())).toLatin1().data();

    //dummy read, to clear buffer
    n_reply->readAll();

    n_reply->deleteLater();
    n_reply = Q_NULLPTR;
    emit finished();
}

after all, I have found the problem and solution. It appeared that the problem was at posting string at POST demand.

The line from POST function:

QByteArray postdata="0000";

Should actually be:

QByteArray postdata("\"0000\"");

Thanks anyway.