Diff with empty changeset causes Differential Revision rendering failure (see: https://secure.phabricator.com/D21490).
Such situation might happen if such empty commit was commited to repository and Phabricator picks it up to close corresponding Differential Revsion (this happened in our instance).
It is possible to also reproduce it with heavily modified arcanist where "emptiness" checks are on purpose removed (D21490 case).

Pre patch generate Diff with empty changeset D21490. Try opening - fails. Apply patch - try opening - works, change as expected is displayed as empty