Can't follow any users - Error saving the notification setting

Steps to reproduce:

1. Login as any simple user
2. Go to another user's profile
3. Under Activity stream, Notifications click on "Not following"
4. Click on "Start following"

Expected results: A "Done" message appears at the bottom of the page and under Notifications the "Not following" changed to "Following".

Actual results: An error occurs at the bottom of the page: 'Error saving the notification setting', which makes following the user not possible.

NOTE: Issue doesn't reproduce on XWiki 15.9 or 14.10.20