I found out the solution! I signed out and I signed in on iCloud and I haven't received that message anymore.
I took a photo with my iPad and, after a while, I got it on streaming photo in iPhoto. Now it works.
P.S.: when I signed out a pop up advise me that if "I'd haven't done a back up of all my contacts I'd have lost them all". It didn't happen. I have already a copy on my iPad so, when I signed in again on iCloud, I found all my contacts on my iMac.