When you try to import a CDStub from the release editor, you got the following result: "An error occured: error".

The reason is that the Javascript Webservice for CDStub is broken:
http://musicbrainz.org/ws/js/cdstub?q=Symphony+No.+9&artist=&tracks=&page=1

Caught exception in MusicBrainz::Server::Controller::WS::js->cdstub_search "encountered object '', but neither allow_blessed nor convert_blessed settings are enabled at local/lib/perl5/JSON/Any.pm line 507."