[Zope] Zope 2.7 - XML export

Hello, I tried to use the XML export feature of an ordered Folder tree but got: Traceback (innermost last): Module ZPublisher.Publish, line 100, in publish Module ZPublisher.mapply, line 88, in mapply Module ZPublisher.Publish, line 40, in call_object Module OFS.ObjectManager, line 502, in manage_exportObject Module OFS.XMLExportImport, line 58, in exportXML Module OFS.XMLExportImport, line 33, in XMLrecord Module Shared.DC.xml.ppml, line 253, in load Module pickle, line 872, in load Module pickle, line 1209, in load_appends AttributeError: List instance has no attribute 'extend' Any hint what might went wrong here? Kind regards Andreas.