None of my photos visible anymore on icloud.com

This used to work just fine until a couple of weeks ago. I regularly take photos with my iPhone 6 (iOS 9.3.1) and they have always been uploaded and visible on icloud.com (Win 7). Unfortunately, they are not visible anymore. I can see that they are uploaded, though. But every photo is replaced by an exclamation mark... On my phone, iCloud Photos is activated.

Would really appreciate some help to resolve this !