issues establishing connection between broker and Qt using Qmqtt

[Yina]

Hello

I am trying to establish a MQTT connection between my app and an external broker with a certificate. . When I try to connect to the host I do get a log on the broker but on QT I get the "QMqttClient::TransportInvalid" error. Anyone know what the issue could be?

Logging Ubuntu mosquitto broker behind nginx proxy:
1675345598: New connection from 172.17.0.1:60662 on port 8883.
1675345598: Client <unknown> closed its connection.

It does work with other applications like MqttBox.

Here is the code I use:

    QFile certFile(":/ca.cer");
    certFile.open(QIODevice::ReadOnly);
    client = new QMqttClient(this);
    QSslCertificate cert = QSslCertificate(&certFile, QSsl::Der);
    QSslConfiguration conf;
    conf.setProtocol(QSsl::TlsV1_2);
    conf.setCaCertificates({cert});
    QSslConfiguration::setDefaultConfiguration(conf);


    QObject::connect(client, &QMqttClient::stateChanged, [](QMqttClient::ClientState state){
        if(state == QMqttClient::Disconnected)
            qDebug() << " State: Disconnected";
        else if(state == QMqttClient::Connecting)
            qDebug() << " State: Connecting";
        else if(state == QMqttClient::Connected)
            qDebug() << " State: Connected";
    });

    QObject::connect(client, &QMqttClient::errorChanged, [](QMqttClient::ClientError error){
         qDebug() << "error Mqtt: " << error;

     });

    client->setHostname(hostName);
    client->setPort(portName);
    client->setUsername(userName);
    client->setPassword(passWord);
    client->setClientId(clientId);
    client->setProtocolVersion(QMqttClient::MQTT_3_1_1);

    client->connectToHostEncrypted(conf);```

Kind regards
Yina