"Requested operation is presently disabled"

[Helge]

I integrated the Visio 2007 ActiveX-Control into a WSS-WebPart.
I build a component that runs in IE and is initialized with the
ActiveX-Control.

For opening a stencil I do:

Page currentPage = m_document.Pages[1];
m_stencil =
m_drawingControl.Document.Application.Documents.OpenEx(stencilFile,
(short)VisOpenSaveArgs.visOpenDocked);
m_stencilWindow = m_document.OpenStencilWindow();
m_stencilWindow.Activate();

Following error occours:

System.Runtime.InteropServices.COMException (0x86DB089A):
Requested operation is presently disabled
in Microsoft.Office.Interop.Visio.DocumentClass.OpenStencilWindow()

It's strange, that everyting works fine outside a WebPart!

Can you help me ?

Thanx!