This often happens when people enable backup on the "Whatsapp" image folder.
Typically it helps to follow the following steps:
Delete the App-data of the Google Photos app as explained here: https://support.google.com/photos/answer/6174018?hl=en&ref_topic=6156061
Do the same for the Google Play Services, and also uninstall the updates of this app
Restart the phone and update the Play Services
It should now be possible to add the folder
Shortened link to this page: https://goo.gl/dMwxWI