Failure in /accounts/confirm/*token* due to multiple authentication backends

I got this traceback:

Traceback:

File "/usr/lib/python3/dist-packages/django/contrib/auth/__init__.py" in login
  143.         backend = backend or user.backend
File "/usr/lib/python3/dist-packages/django/utils/functional.py" in inner
  239.         return func(self._wrapped, *args)

      During handling of the above exception ('AnonymousUser' object has no attribute 'backend'), another exception occurred:

File "/usr/lib/python3/dist-packages/django/core/handlers/exception.py" in inner
  41.             response = get_response(request)
File "/usr/lib/python3/dist-packages/django/core/handlers/base.py" in _get_response
  187.                 response = self.process_exception_by_middleware(e, request)
File "/usr/lib/python3/dist-packages/django/core/handlers/base.py" in _get_response
  185.                 response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/usr/lib/python3/dist-packages/django/views/generic/base.py" in view
  68.             return self.dispatch(request, *args, **kwargs)
File "/usr/lib/python3/dist-packages/django/views/generic/base.py" in dispatch
  88.         return handler(request, *args, **kwargs)
File "/srv/tracker.debian.org/distro-tracker/django_email_accounts/views.py" in post
  192.         return super(SetPasswordMixin, self).post(request, confirmation_key)
File "/usr/lib/python3/dist-packages/django/views/generic/edit.py" in post
  183.             return self.form_valid(form)
File "/srv/tracker.debian.org/distro-tracker/django_email_accounts/views.py" in form_valid
  180.         login(self.request, user)
File "/usr/lib/python3/dist-packages/django/contrib/auth/__init__.py" in login
  150.                 'You have multiple authentication backends configured and '
Exception Type: ValueError at /accounts/confirm/50d...
Exception Value: You have multiple authentication backends configured and therefore must provide the
`backend` argument or set the `backend` attribute on the user.
Request information:
USER: AnonymousUser

GET: No GET data

POST:
password2 = '...'
password1 = '...'
csrfmiddlewaretoken = '...'

FILES: No FILES data

COOKIES:
csrftoken = '...'

I stripped private information such as tokens and passwords. I have the full details in my mail archive (msg id 20180325192857.24487.23700@ticharich.debian.org).