I am using social login in my Django app. So, I have added additional backends in my settings.py file.
settings.py
AUTHENTICATION_BACKENDS = [ \'django.
Came here looking for this error. Our stack is django-oscar + wagtail. It turns out we removed oscar.apps.customer.auth_backends.EmailBackend from our AUTHENTICATION_BACKENDS. Putting it back solved the issue.
oscar.apps.customer.auth_backends.EmailBackend
AUTHENTICATION_BACKENDS