In Dynamics AX 2009 several Forms exists that have a Web browser control to display either dynamically created HTML content or show certain web pages based on the context of a selected record in a Grid. The Web browser control enriches the user experience this way.
However sometimes it might happen when you open such a Form or change the record context so a different HTML page gets displayed the Dynamics AX client terminates. If the Dynamics AX Client terminates or not mainly depends what kind of content is being displayed in the Web browser control.
HTML pages do often also host interactive objects to enrich their user experience. If on such a HTML page SilverLight content is embedded, event if it is just for advertisements, exactly the described issue will happen. Background of this issue is the misinterpretation of a WM_USER Windows Message by the Dynamics AX Client.