Do you have either iThemes Security or All in One Security and Firewall installed? It seems both block the calls for the HiDPI (Retina) images. They are created and cached on demand.
All in One Security and Firewall seems to have some sort of function to use a custom admin URL (reported by a user). After they disabled that, the HiDPI images loaded fine. Not sure why that would be the case because no admin URLs are used for the images.
iThemes Security has a feature to Filter Suspicious Query Strings in the URL when that feature is disabled the HiDPI images will load fine (reported by a user). Since the query vars are actually registered with WordPress my guess at this time iThemes Security is not checking against the list of registered query vars before flagging/bocking them as suspicious.