Error loading XAML file in designer

When I open up my XAML file from a WPF 4.0 application in the VS 2010 designer, I get this error message:

Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt.
   bei Microsoft.Expression.Platform.WPF.InstanceBuilders.DocumentNodeObjectReader.get_Value()
   bei System.Xaml.XamlWriter.WriteNode(XamlReader reader)
   bei System.Windows.Markup.WpfXamlLoader.TransformNodes(XamlReader xamlReader, XamlObjectWriter xamlWriter, Boolean onlyLoadOneNode, Boolean skipJournaledProperties, Boolean shouldPassLineNumberInfo, IXamlLineInfo xamlLineInfo, IXamlLineInfoConsumer xamlLineInfoConsumer, XamlContextStack`1 stack, IStyleConnector styleConnector)
   bei System.Windows.Markup.WpfXamlLoader.Load(XamlReader xamlReader, IXamlObjectWriterFactory writerFactory, Boolean skipJournaledProperties, Object rootObject, XamlObjectWriterSettings settings, Uri baseUri)
   bei System.Windows.Markup.WpfXamlLoader.Load(XamlReader xamlReader, Boolean skipJournaledProperties, Uri baseUri)
   bei System.Windows.Markup.XamlReader.Load(XamlReader xamlReader, ParserContext parserContext)
   bei System.Windows.Markup.XamlReader.Load(XamlReader reader)
   bei Microsoft.Expression.Platform.WPF.InstanceBuilders.FrameworkTemplateInstanceBuilder.InstantiateTemplate(IDocumentRoot xamlDocument, TemplateSerializerContext context)
   bei Microsoft.Expression.DesignModel.InstanceBuilders.FrameworkTemplateInstanceBuilderBase`7.Instantiate(IInstanceBuilderContext context, ViewNode viewNode)
   bei Microsoft.Expression.DesignModel.Core.ViewNodeManager.Instantiate(ViewNode viewNode)

I've tried to analyse it by the instructions in the help, but debugging the VS process didn't work and I cannot determine the designer mode in my markup extension class. Also this exception message is not helpful. Where can I go on searching now?

I've tried to create a small test case application containing only the offending code, but everything works fine there. I could not create a working testcase yet to demonstrate the error.