I have latest updates to aperture applied. I share a library with iphoto. If i try to enable photostream from preferences a spinning beachball starts and the application becomes unusable. I have to force quit. Restarting doesnt fix it. I have repaird and rebuilt the database. I also notice that while the application shows not responding their is also a photstream process running that is using up 100% of cpu? in the past i have tried delting preference files to no avail. Really need help here.
Looking at the Photstream process that you mentioned in your first post it appears that it accesses a few different files/folders in your home Library folder.
I would try turning off Photostream for the account that is not working
(System Preferences->iCloud->Photostream) then moving
to the Desktop and then enabling Photstream and then try it in Aperture.