Iphoto stopped uploading to Photostream

My Photostream life was going great. All the devices were uploading and sharing photos seamlessly.

Then one day I noticed that iPhoto had mysteriously deleted all the pictures from my Christmas DSLR photo session.

Since then, the Photostream is working fine for the iPads and iPhones... but my iPhoto is only receiving photos on its photostream, not uploading.

It says it is trying to upload.. but is permanently stuck in "uploading 1 picture".

I have tried:

1. Reinstalling iPhoto.

2. Shutting down Photostream

3. Logging on and off iCloud

4. Tried manually selecting pictures to upload

Nothing changed. I've been looking for a solution but apparently I'm alone with this problem.