... per eliminare la chiamata a getCenterPane dal costruttore, ho fatto questa piccola modifica:

in AbsDialogOK_ANN
codice:
publicAbsDialogOK_ANN(String title){
        super(newJFrame(),title,true);
        this.setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
        this.setResizable(false);
//      this.setContentPane(this.getJCP());
//      finalDimension screenSize = java.awt.Toolkit.getDefaultToolkit().getScreenSize();
      //this.setSize(newDimension(screenSize.width /2,screenSize.height /2));
//      this.setLocation((screenSize.width -this.getWidth())/2,(screenSize.height -this.getHeight())/2);
//      this.setVisible(true);
    }

protected void init(){
this.setContentPane(this.getJCP());
}
dunque... in LogInGUI
codice:
public LogInGUI(){
  super("Login");
  this.init();

}

@Override
protected void init(){
    super.init();
    final Dimension screenSize = java.awt.Toolkit.getDefaultToolkit().getScreenSize();
    this.setSize(newDimension(screenSize.width /2,screenSize.height /2));
    this.setLocation((screenSize.width -this.getWidth())/2,(screenSize.height -this.getHeight())/2);
    this.setVisible(true);
}
sembra si siano risolti i problemi...
Attendo comunque eventuali consigli