@Rule public TemporaryFolder testFolder = new TemporaryFolder();

    private void testExport2File(CDOTransaction transaction) throws IOException {
        CDOResourceFolder folder = transaction.createResourceFolder("/serializationtest");
        CDOResource res = folder.addResource("someres");

        export2File(testFolder.newFile(), EcoreUtil.copy(folder)); //<<<< this still works
        Annotation mockupObject = EtypesFactory.eINSTANCE.createAnnotation();
        mockupObject.setSource("somesource");
        res.getContents().add(mockupObject);
        export2File(testFolder.newFile(), EcoreUtil.copy(folder)); //<<<<< exception here
    }

    private void export2File(File destination, EObject object) throws IOException {
        String fileName = destination.getAbsolutePath();
        System.out.println("Exporting to: " + fileName);
        Resource.Factory.Registry reg = Resource.Factory.Registry.INSTANCE;
        Map<String, Object> m = reg.getExtensionToFactoryMap();
        m.put("key", new XMIResourceFactoryImpl());
        ResourceSet resSet = new ResourceSetImpl();
        Resource resource = resSet.createResource(URI.createFileURI(fileName));
        resource.getContents().add(object);
        resource.save(Collections.EMPTY_MAP);
    }