In src/librygel-server/rygel-item-updater.vala, Rygel.ItemUpdater::fetch_object() :-

        ...
        } else if (media_object.parent.restricted) {
            var msg = _("Metadata modification of object %s being a child " +
                        "of restricted object %s not allowed");
                     
            throw new ContentDirectoryError.RESTRICTED_PARENT
                                        (msg,
                                         media_object.id,
                                         media_object.parent.id);
        }
        ...

The message is only partially marked for translation in the POT:

#: ../src/librygel-server/rygel-item-updater.vala:195
#, c-format
msgid "Metadata modification of object %s being a child "
msgstr ""