check, if you have exceeded any of the limits for shared Photo Streams given here:
If it is not a question of limits, have a look at the folder
This folder, inside your User Library, stores all image files to be uploaded to your Photo Stream and the Shared Photo Streams. If one of the images files is in an incompatible format, or did not download correctly, the Photo Stream may stop working. So check the files you tried to add most recently to your shared stream, and remove them if necessary.
The user library is hidden. To reveal it, use the Finder's "Go" menu.
- Click somewhere on your empty Desktop to bring the Finder forward, then you'll see "Go" in the Finder's main menu bar.
- Then hold down the options key ⌥ until "library" appears in the drop down menu. Click Library to open.
- When the Library window appears, select "Application Support", then "iLifeAssetManagement" and "Assets".
Thanks, the folder seemed to have everything in order. I also was within all of the limits for iCloud.
I did manage to solve the issue and here is how. I went into System Preferences/ Users and Groups. I added another iCloud ID to the list and switched that user to the active account. I then went back and launched Aperture. Aperture recognized the change. I then quit out of Aperture, went back to the Users and Groups and switched back to my iCloud ID. When I went back into Aperture, it recognized that I was the active iCloud ID and the shared streams came up properly and then I was able to successfully add additional shared streams.
Everything is back and working properly. Hope Apple takes note on this and issues a fix at some point.