I'm making a GET request on a local address to get data from an Arduino(ESP8266) board. The board hosts a server, and sends to the client the content of a flle in its filesystem. For reference, this is the code on the Arduino board: 

WiFiServer server;
//...
WiFiClient client = server.available();  
//...
client.print("HTTP/1.1 200 OK\n\n"); // http header
client.write(file);                  // local file

 The content of the file is sent as response to the client. 

This is the Qt code, the download starts when a push button is clicked in the gui: 

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow),
    nam(new QNetworkAccessManager)
{
    ui->setupUi(this);
    connect(nam,&QNetworkAccessManager::finished,[](QNetworkReply* r){
        qDebug()<<QString(r->readAll());
        r->deleteLater();
    });
}

void MainWindow::on_pushButton_clicked()
{
    QNetworkReply* r = nam->get(QNetworkRequest(QUrl("http://192.168.1.103")));
    connect(r,static_cast<void (QNetworkReply::*)(QNetworkReply::NetworkError)>(&QNetworkReply::error),[](QNetworkReply::NetworkError err){
        qDebug()<<err;
    });    
}

I found out that if the file has more than 32763 characters, qt's QNetworkAccessManager::finished is never fired anymore, and also QNetworkReply::NetworkError is never triggered. Chrome and curl have no problem loading the response(even with much more characters), so the problem is not in the Arduino code.

The mentioned maximum number of characters 32763 is quite close to 2^15 so it seems it's not a random value.