Permission error while locking the device when writing to QSerialPort

[Cazzaniga]

Hi,
I'm working on a Qt5.7 application cross-compiled for my device. I added the serialport module to read and write to the serialport.
After opening the QSerialPort, when I try to write to it, the following error message (attached to the thread) appears in the GUI: "Permission error while locking the device", and the message is not sent to the serial port.
This is the way I open and try to write to the serial port:

serialport->setPortName("/dev/ttymxc2");
    serialport->setBaudRate(QSerialPort::Baud9600);
    serialport->setDataBits(QSerialPort::Data8);
    serialport->setParity(QSerialPort::NoParity);
    serialport->setStopBits(QSerialPort::OneStop);
    serialport->setFlowControl(QSerialPort::NoFlowControl);
    //serial->setFlowControl(QSerialPort::HardwareControl);
    if (serialport->open(QIODevice::ReadWrite))
        //opened

serialport->setPortName("/dev/ttymxc2");
    serialport->setBaudRate(QSerialPort::Baud9600);
    serialport->setDataBits(QSerialPort::Data8);
    serialport->setParity(QSerialPort::NoParity);
    serialport->setStopBits(QSerialPort::OneStop);
    serialport->setFlowControl(QSerialPort::NoFlowControl);
    //serial->setFlowControl(QSerialPort::HardwareControl);
    if (serialport->open(QIODevice::ReadWrite))
        //opened

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...

The write section:

QByteArray ba;
ba.resize(50);
...
 const qint64 bytesWritten = serialport->write(ba);

QByteArray ba;
ba.resize(50);
...
 const qint64 bytesWritten = serialport->write(ba);

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Could anyone help to find the solution? What am I missing?
Thank you!