Unable to import any photos into Aperture

Every time I add a photo to 3.5.1 Aperture it gets flagged with a warning image and then removed (disappears). Basically it fails to import, but doesn't complain about it. I just rebuilt my Aperture library and see the same results. There is no real user-level warning that I see. Similarly trying covert the library to iPhoto and import a photo fails in the same silent way.

I have my library in the shared folder with user-group permissions so I can share it between multiple users and I access it regularly that way remotely through network. This has been working (although not amazingly) for some months and just started with this behavior some weeks ago.

This is on a 2012 Mac Mini running Yosemite.

Can anyone help?