Lastest version of CustomPress 184.108.40.206 Network Activated. Causes Media Library to appear empty even though it reports the correct number of images that are actually there when you try to add a new image from inside a new post (see screenshots). Might be a similar issue as this dealing with custom taxonomies:
Deactivating CustomPress the add image from Media Library works as expected.
Screen 1 – CustomPress active, no images appear in Media Library
Screen 2 – CustomPress deactivated, images appear in Media Library