Aperture iCloud streams all empty

I recently upgraded to Yosemite and Aperture 3.6.

I can't get iCloud to work in Aperture anymore. When I enable iCloud in Aperture, all my old iCloud streams, as well as all streams of Family and Friends', show up empty. All streams are there, but empty. Aperture summarizes: "38 streams, 0 photos".

I can publish new streams from Aperture, but seemingly not access existing streams.

Is this something I can workaround?

I've tried opening Photos (taking care not to open or import my Aperture library) and all streams and photos, including the one I created in Aperture, showed up there. After switching iCloud back in Aperture, all streams are still empty, and the one I created in Aperture is gone!

I would appreciate any help or suggestion.