http://www.musicbrainz.org/search?query=don+giovanni&type=work returns "Internal Server Error".
http://www.musicbrainz.org/search?query=le+nozze+di+figaro&type=work succeeds.

Works (intended to be) returned by the first search have seen a lot of editing recently. Perhaps the problem could be due to the works having open edits, or due to the search trying to return works that have been merged.