Perhaps the shared album contains unreadable files (image or videos).
Have you tried to disable iCloud Photo Sharing in the System Preferences > iCloud, then restart the Mac to clear the caches, then enable iCloud Photo Sharing in the System Preferences > iCloud and in Photos again?
If that does not help, delete the cached files and folders for the shared photo streams manually.
They are stored in your User library in this folder:
Navigate first o this folder, by by pasting the path below in the Finder > Go > Goto Folder textfield
~/Library/Containers/com.apple.cloudphotosd/Data/Library/Application Support
In the Finder window that opens, navigate to this subfolder:
/com.apple.cloudphotosd/services/com.apple.photo.icloud.sharedstreams/assets
Copy the folder assets for safekeeping to an external drive, so you have a copy of the media that used to be in the shared stream, then delete the folder com.apple.photo.icloud.sharedstreams/assets.
Restart the mac again and wait for the photos and videos to download again from iCloud.