i had this issue in ubuntu 14.04 and with both python 2.x and 3.x.
all of this is because when creating new user in mezzanine,
(specially first user with createdb management command which is admin user)
sitepermission of the user dose not add automatically.
so when create new project log in to admin and edit admin user and select the only available site "
127.0.0.1:8000" in sitepermission and save.
so SitePermission.objects.count() equals1
do this procedure for every other staff user.
and if there are two sitepermission inline,
only selecting the first one solve the issue.