On my Windows 7 64, I've had this issue since upgrading to iCloud 4. Sometimes I would open iCloud Photos on my Windows laptop and it would indicate not signed in.
After reading this blog, I tried all of the suggestions. I clicked the Add Photo link in the empty photos folder, and I could see that all of my old Photo Stream photos, sitting in the folder. I actually had two folders and I looked in both and saw all of my old synced photos. I didn't add any of them.
So I had an idea based on a few of the posts - I tried it and I just got iCloud 4 to work. Here's what I did...pretty simple - don't know why I didn't try this sooner.
- signed out of and uninstalled iCloud
- moved the user/My Pictures/iCloud Photos folder to my desktop
- moved the user/My Pictures/Photo Stream folder to my desktop
- there were two folders, as you can see above - not sure if it was because of the uninstall (there was a message that the photos were saved somewhere, and I just clicked OK knowing that they were on the laptop).
- reinstalled a fresh copy of iCloud 4
- rebooted
IMMEDIATELY after reinstalling iCloud 4 once I moved the folders, it began updating with the current photos from my iPad/iPhone.
Hope this works for you!