Receiving QNetworkReply::ProtocolFailure to GET request

I apologize for the lengthy post. I am attempting to create a remote program that will communicate with a main program running on SBC. The remote needs to run on a browser. The remote has a UI to send control information as well as to request updates from the main application (same as operating an oscilloscope through a Web interface).

When the remote is compiled as a desktop application, I can receive the data from the main application properly. However, when I compile the remote application using Webassembly, I get QNetworkReply::ProtocolFailure every time I receive a reply from my main app. Monitoring the HTTP communication, using Wireshark, I see that the GET request and reply are sent properly.

I create two small server-client applications that can easily reproduce the problem. The client had QTextEdit Widget to output debug information.

My test server: main.cpp:

#include "mainwindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

mainwindow.h

ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QTcpServer>
#include <QHttpServer>

QT_BEGIN_NAMESPACE
namespace Ui {
class MainWindow;
}
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private:
    Ui::MainWindow *ui;

    int const NETWORK_PORT_SERVER   {12345};
    int const NETWORK_PORT_CLIENT   {23456};

    QTcpServer *tcpServer;
    QHttpServer hServer;
};
#endif // MAINWINDOW_H

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    tcpServer = new QTcpServer(this);
    tcpServer->listen(QHostAddress("192.168.86.48"), NETWORK_PORT_SERVER);
    hServer.bind(tcpServer);

    hServer.route("/", QHttpServerRequest::Method::Options, []()
    {
        QHttpServerResponse res(QHttpServerResponse::StatusCode::NoContent);
        res.addHeader("Allow", "*");
        res.addHeader("Access-Control-Allow-Origin", "*");
        res.addHeader("Access-Control-Allow-Language", "*");
        res.addHeader("Access-Control-Allow-Encoding", "*");
        res.addHeader("Access-Control-Allow-Method", "*");
        res.addHeader("Access-Control-Allow-Headers", "*");
        return res;
    });

    hServer.route("/", QHttpServerRequest::Method::Get, []()
    {
        QString testAnswer = "TEST REPLY";
        QHttpServerResponse res(testAnswer, QHttpServerResponse::StatusCode::Ok);
        return res;
    });
}

MainWindow::~MainWindow()
{
    delete ui;
}

My test client: main.cpp:

#include "mainwindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

mainwindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QNetworkAccessManager>
#include <QNetworkReply>
#include <QtHttpServer>

QT_BEGIN_NAMESPACE
namespace Ui {
class MainWindow;
}
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private:
    Ui::MainWindow *ui;

    int const NETWORK_PORT_SERVER   {12345};
    int const NETWORK_PORT_CLIENT   {23456};

    QNetworkAccessManager * manager;
    QNetworkRequest req;

private slots:
    void serverDataReceived(QNetworkReply *);
};
#endif // MAINWINDOW_H

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    ui->debugInfo->append("Program Started");

    manager = new QNetworkAccessManager(this);
    connect(manager, SIGNAL(finished(QNetworkReply*)), this, SLOT(serverDataReceived(QNetworkReply*)));
    QUrl contAdress(QString("http://192.168.0.84:%1").arg(NETWORK_PORT_SERVER));
    req.setUrl(contAdress);
    manager->get(req, "Test");
}

void MainWindow::serverDataReceived(QNetworkReply * rep)
{
    ui->debugInfo->append("Reply Received");
    QVariant status = rep->attribute(QNetworkRequest::HttpStatusCodeAttribute);
    ui->debugInfo->append(status.toString());
    ui->debugInfo->append(QString("Network Error: %1").arg(static_cast<int>(rep->error())));
    ui->debugInfo->append(rep->readAll());
}

MainWindow::~MainWindow()
{
    delete ui;
}

Does anyone know why this happens and how to resolve this?

The solution to the problem is to add the header res.addHeader("Access-Control-Allow-Origin", "*"); to the HTTP server;

hServer.route("/", QHttpServerRequest::Method::Get, []()
    {
        QString testAnswer = "TEST REPLY";
        res.addHeader("Access-Control-Allow-Origin", "*");  // Added line
        QHttpServerResponse res(testAnswer, QHttpServerResponse::StatusCode::Ok);
        return res;
    });

After adding this line, the browser forwarded the reply to my application.