Comment 25 for bug 1210253

I'm not sure if this is too late but I was getting the same error everytime that I started an instance of the horizon layer. I discovered that I was using the version 1.6 of Django and it has some classes deprecated(https://docs.djangoproject.com/en/dev/internals/deprecation/), specially django.conf.urls.defaults. My workaround was to downgrade to 1.5 version and finally works.