If element_name was not defined, constructor takes a class name (this line). But get_class returns not only the class name, but the whole namespaces chain. To fix that, on lines 164-168, $element_name gets cleaned of namespaces, but element_name_plural doesn't. And as element_name_plural is used in URL's, they won't work.
Fixed element_name_plural when namespaces are used, closes #16
Thanks for catching this!