StackPanel not rendered correctly

Hi all,

I wonder why my stackpanel in the datagrid column header is displayed like this:

"System.Windows.Controls.StackPanel".

I know that this happens if there is no data template for poco classes, but the StackPanel is a WPF control and all it's childs, too.

Here is the XAML of the stackpanel:

<StackPanel
  Orientation="Horizontal"><ContentControl>Rule Name</ContentControl><Image
    Source="/DataGridContextMenuFilter;component/Images/filter.png" /></StackPanel>

Can anybody point me into the right direction?

Thank you