How to update text in QLineEdit using setText() inside slot function [Qt5.4]

Question

I am just starting out with Qt (5.4 to be precise), using it to build a GUI for a C++ project to control some hardware. Now my problem is, I cant update any QLineEdit (or QLabel, but this seems to be the same problem) text displayed using its setText() method. By 'update' I mean it works once when called from the main window constructor but then no more. I am using setText() from within the slot functions I defined. Even using repaint() right afterwards didn't do anything.

I must be doing something horribly wrong here...

Update: this is indeed correct, my own code is causing problems with GUI thread

Here is an example

main.cpp

int main( int argc, char* argv[] ) {
    QApplication app(argc, argv);
    CmainWindow mainApp;
    mainApp.show();
    return app.exec();
}

CMainWindow.h

class CMainWindow : public QMainWindow, private Ui::MainWindow {
    Q_OBJECT
    Q_DISABLE_COPY( CMainWindow )
public:
    CMainWindow( QWidget *parent = 0 );
    ~CMainWindow();
public slots:
    void slotButtonConnectClicked(void);
protected:
    void setConnections();
private:
    AbstractReader_t* pReader;
    QString* ErrorString; // conversion mule for std::string 
);

CMainWindowConnections.cpp

void CMainWindow::setConnection(void) {
    Q_CHECK_PTR( pushButton_Connect )
    connect( pushButton_Connect, SIGNAL( clicked() ), this, SLOT( slotButtonConnectClicked()) );
}

CMainWindow.cpp

CMainWindow::CMainWindow( QWidget *parent ) : QMainWindow(parent) {
    pReader = NULL;
    ErrorString = new QString("GUI Initialized"); // just something to test
    setupUi(this);
    setConnections();
    lineEdit_Error->setText(*ErrorString); // this works !
}

void CMainWindow::slotButtonConnectClicked( void ) {
     lineEdit_Error->setText("Initialising..."); // this wont show
     // some of my own code here caused problem
} //nope, not even after here