When calling DoModal of a CDialog derived object (an error message dialog),
it is not shown on the screen; the event OnShowWindow (WM_SHOWWINDOW) is not even reached.
The parent window is a CFrameWnd derived class having the attribute "DS_SYSMODAL",
and having one child window displayed.
When this child window is not yet created, the error message dialog is displayed;
but after creation of the child the error box isn't shown any more.
Even when the child window is parent of the error dialog, the error is invisible.
The error message comes out of a worker thread.