Cannot connect to the device with QSerialPort

Question

I develop Qt application in C++ under manjaro linux. The goal is to connect with a qC (Nucleo-L476RG Board) and receive data from accelerometer. App can find device on /dev/ttyACM0, but when i try to connect it fails with error "QSerialPort::DeviceNotFoundError". Checking errorString() gives "No such file or directory", but it still can find it in /dev. I'm in the right group to read the file: [1]: https://i.stack.imgur.com/CG1yZ.png I use Qt v.6.2.4, but have build the code with Qt v.5.15.3. I tried to run another person's app that does the same and works under ubuntu, but it didn't work. Here is my code with the searching and connecting methods. Method addToLogs() just prints logs in textEdit.

void MainWindow::addToLogs(QString message)
{
    QString currDateTime = QDateTime::currentDateTime().toString("yyyy.MM.dd hh:mm:ss");
    ui->textEditLogs->append(currDateTime + "\t" + message);
}

void MainWindow::on_pushButtonSearch_clicked()
{
    ui->comboBoxDevices->clear();
    this->addToLogs("Szukam urządzeń...");

    QList<QSerialPortInfo> devices;
    devices = QSerialPortInfo::availablePorts();
    if(devices.count() > 0)
    {
        for(int i=0; i<devices.count(); i++)
        {
            this->addToLogs(devices.at(i).portName());// + " " + devices.at(i).description());
            //ui->comboBoxDevices->addItem(devices.at(i).systemLocation());//portName());
            ui->comboBoxDevices->addItem(devices.at(i).portName());
        }
    }
}

void MainWindow::on_pushButtonConnect_clicked()
{
    if(ui->comboBoxDevices->count() == 0)
    {
        this->addToLogs("Nie wykryto żadnych urządzeń!");
        return;
    }
    if(this->device->isOpen())
    {
        this->addToLogs("Port jest już otwarty!");
        return;
    }

    QString portName = ui->comboBoxDevices->currentText();

    //this->device->setPortName(portName);
    this->device->setPortName("/dev/ttyACM0");
    qDebug() << this->device->portName();
    if(device->open(QSerialPort::ReadWrite))
    {
        this->device->setBaudRate(QSerialPort::Baud9600);
        this->device->setParity(QSerialPort::NoParity);
        this->device->setDataBits(QSerialPort::Data8);
        this->device->setStopBits(QSerialPort::OneStop);
        this->device->setFlowControl(QSerialPort::NoFlowControl);

        this->addToLogs("Połączono z urządzeniem " + portName);
    }
    else
    {
        this->addToLogs("Otwarcie portu szeregowego się nie powiodło!");
        this->addToLogs(device->errorString());
        qDebug() << this->device->error();
    }
}

I will be thankful for help, cause i've benn sitting on it for last 2 or 3 weeks and nothing works.