//Panel für CatPartname
StackPanel pnl_01 = this.Resources["lbxitem01_pnlCatPartName"] as StackPanel; _
  //funktioniert
if (pnl_01 != null)
{
   //Label Partname: Cap + Value
   strCaption = "Part name:";
   Label TestlblPartnameCap = pnl_01.FindName("lblPartnameCap") as Label; _
     //liefert null
   if (TestlblPartnameCap != null) TestlblPartnameCap.Content = strCaption;// _
   funktioniert nicht nicht
                    ....

<Window x:Class="Wpf_CATIA_3D_Print.wndCatia"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Catia Evalution" Height="800" Width="800"
    Background="{DynamicResource ResourceKey = Global_WindowBackground}">
 
    <Window.Resources>
        <!--Panel für CatPartname-->
        <StackPanel x:Key="lbxitem01_pnlCatPartName" Orientation="Horizontal">
            <!--Label Partname Caption + Value -->
            <Label x:Name="lblPartnameCap" Style="{StaticResource" & _
              "MyLabelCaption}"></Label>
            <Label x:Name="lblPartnameVal" Style="{StaticResource" & _
            "MyLabelValue}"></Label>
 </StackPanel>
    </Window.Resources>