Dialogs and Pop-Up Windows not corectly displayed with Timer caling update on another widget in background frequently

Wenn porting some applications from 4.5.2 to 4.6.0 I realized a problem when I have frequent updates triggered by a timer in my main window to some childs.
When the trigger is on pop-up's like drop down boxes are not shown.
For some Dialogs I can resolve the problem by not caling exec(); but caling setModal(true); and show();