Hello Wagner,
thank You for Your support. Your hint made me read the documentation over and over again - and that's when I found what was missing:
I have the xml documentation files in a subdirectory of my project and that's when I need the second parameter of the method:
TXDataModelBuilder.LoadXmlDoc(XDataServer.Model, 'C:\MyXMLFiles');
Now it works as expected.
Thanks again
Harald