OK, solved. Here is the job I've done:

Qt Code: 
  1. // Form Header (frmSetBackupDir class)
  2.  
  3. // Added
  4. protected:
  5.    bool eventFilter(QObject *obj, QEvent *event);
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Qt Code: 
  1. // Form Implementation
  2.  
  3. // Added in the CTOR
  4.    ui->lneDirBU->installEventFilter(this);
  5.    ui->lneDirBU->setAcceptDrops(true);
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Qt Code: 
  1. bool frmSetBackupDir::eventFilter(QObject *obj, QEvent *event)
  2. {
  3.    if (event->type() == QEvent::DragEnter)
  4.    {
  5.       QDragEnterEvent *tDragEnterEvent = static_cast<QDragEnterEvent *>(event);
  6.       tDragEnterEvent->acceptProposedAction();
  7.  
  8.       return true;
  9.    }
  10.    else if (event->type() == QEvent::DragMove)
  11.    {
  12.       QDragMoveEvent *tDragMoveEvent = static_cast<QDragMoveEvent *>(event);
  13.       tDragMoveEvent->acceptProposedAction();
  14.  
  15.       return true;
  16.    }
  17.    else if (event->type() == QEvent::Drop)
  18.    {
  19.       QDropEvent *tDropEvent = static_cast<QDropEvent *>(event);
  20.       tDropEvent->acceptProposedAction();
  21.  
  22.       qDebug() << "OK, execute your task!";
  23.  
  24.       return true;
  25.    }
  26.    else
  27.    {
  28.        // standard event processing
  29.        return QObject::eventFilter(obj, event);
  30.    }
  31. }
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It seems to work. Better implementations are well accepted!