issues establishing connection between broker and Qt using Qmqtt - c++

Question

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);
The nginx configuration for the MQTT broker.
stream {
##
# Secure port
##
upstream google_mqtts {
server 127.0.0.1:8883; # OUT: 8884 -> IN: 8883
}
server {
listen 8884;
proxy_pass google_mqtts;
ssl_protocols TLSv1 TLSv1.1 TLSv1.2 TLSv1.3; # Dropping SSLv3
ssl_prefer_server_ciphers on;
##
# Logging Settings
##
#access_log /var/log/nginx/access_mqtt.log;
#error_log /var/log/nginx/error_mqtt.log;
}
}
Kind regards
Yina