connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(ftp_connect()));
    connect(&ftp, SIGNAL(commandFinished(int,bool)), this, SLOT(ftp_state(int, bool)));

void MainWindow::ftp_connect()
{
    ftp.connectToHost(ui->lineEdit_ftp_adr->text().simplified(), ui->lineEdit_ftp_port->text().toInt());
    ftp.login(ui->lineEdit_ftp_user->text().simplified(), ui->label_ftp_pass->text().simplified());
}
void MainWindow::ftp_state(int, bool error)
{
    if (ftp.currentCommand() == QFtp::ConnectToHost)
    {
        if(!error)
        {
            qDebug() << "Соединение с хостом установлено";
        }
       qDebug() << ftp.errorString();
    }
}

"Unknown error"
QAbstractSocket::connectToHost() called when already looking up or connecting/connected to "192.168.1.1"

void MainWindow::ftp_connect2()
{
    QUrl url("ftp://" + ui->lineEdit_ftp_adr->text().simplified() + "/1");
    url.setPort(ui->lineEdit_ftp_port->text().toInt());
    url.setUserName(ui->lineEdit_ftp_user->text().simplified());
    url.setPassword(ui->lineEdit_ftp_pass->text().simplified());
    QNetworkRequest request(url);
    QNetworkAccessManager *manager = new QNetworkAccessManager;
    QNetworkReply *reply = manager->get(request);
    connect(reply, SIGNAL(error(QNetworkReply::NetworkError)), this, SLOT(finishedSlot(QNetworkReply::NetworkError)));
}
void MainWindow::finishedSlot(QNetworkReply::NetworkError networkError)
{
     qDebug()<< networkError;
}