Django - LoginView: 'AnonymousUser' object has no attribute '_meta'

I want to use Djangos' built-in LoginView. It works fine until I login a user with a correct password (If the password is wrong it works). Then I get this error:

'AnonymousUser' object has no attribute '_meta'

I already added AUTHENTICATION_BACKENDS = ['django.contrib.auth.backends.ModelBackend'] to my settings.py and I looked at Django login AttributeError: 'AnonymousUser' object has no attribute '_meta' and on other webpages but they couldn't help me.

MIDDLEWARE = [
     # Must have middlewares
    'django.middleware.security.SecurityMiddleware',
    'django.contrib.sessions.middleware.SessionMiddleware',
    'django.middleware.common.CommonMiddleware',
    'django.middleware.csrf.CsrfViewMiddleware',

    'django.contrib.auth.middleware.AuthenticationMiddleware',
    'django.contrib.messages.middleware.MessageMiddleware',


    'django.middleware.clickjacking.XFrameOptionsMiddleware',
    'simple_history.middleware.HistoryRequestMiddleware',
    'admin_reorder.middleware.ModelAdminReorder',

    'django.middleware.cache.UpdateCacheMiddleware',
    'django.middleware.common.CommonMiddleware',
    'django.middleware.cache.FetchFromCacheMiddleware',

     # Custom middlewares
    'utils.Middleware.RangesMiddleware.RangesMiddleware',
    'utils.Middleware.AdminAccessMiddleware.AdminAccessMiddleware',
    'apps.log_request.middlewares.RequestLogMiddleware',
]

.

Internal Server Error: /auth/anmelden
Traceback (most recent call last):
  File "C:\Program Files\Python37\lib\site-packages\django\core\handlers\exception.py", line 34, in inner
    response = get_response(request)
  File "C:\Program Files\Python37\lib\site-packages\django\core\handlers\base.py", line 115, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "C:\Program Files\Python37\lib\site-packages\django\core\handlers\base.py", line 113, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "C:\Program Files\Python37\lib\site-packages\django\utils\decorators.py", line 142, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "C:\Program Files\Python37\lib\site-packages\django\views\generic\base.py", line 71, in view
    return self.dispatch(request, *args, **kwargs)
  File "C:\Program Files\Python37\lib\site-packages\django\utils\decorators.py", line 45, in _wrapper
    return bound_method(*args, **kwargs)
  File "C:\Program Files\Python37\lib\site-packages\django\views\decorators\debug.py", line 76, in sensitive_post_parameters_wrapper
    return view(request, *args, **kwargs)
  File "C:\Program Files\Python37\lib\site-packages\django\utils\decorators.py", line 45, in _wrapper
    return bound_method(*args, **kwargs)
  File "C:\Program Files\Python37\lib\site-packages\django\utils\decorators.py", line 142, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "C:\Program Files\Python37\lib\site-packages\django\utils\decorators.py", line 45, in _wrapper
    return bound_method(*args, **kwargs)
  File "C:\Program Files\Python37\lib\site-packages\django\views\decorators\cache.py", line 44, in _wrapped_view_func
    response = view_func(request, *args, **kwargs)
  File "C:\Program Files\Python37\lib\site-packages\django\contrib\auth\views.py", line 61, in dispatch
    return super().dispatch(request, *args, **kwargs)
  File "C:\Program Files\Python37\lib\site-packages\django\views\generic\base.py", line 97, in dispatch
    return handler(request, *args, **kwargs)
  File "C:\Program Files\Python37\lib\site-packages\django\views\generic\edit.py", line 142, in post
    return self.form_valid(form)
  File "C:\Program Files\Python37\lib\site-packages\django\contrib\auth\views.py", line 90, in form_valid
    auth_login(self.request, form.get_user())
  File "C:\Program Files\Python37\lib\site-packages\django\contrib\auth\__init__.py", line 126, in login
    request.session[SESSION_KEY] = user._meta.pk.value_to_string(user)
  File "C:\Program Files\Python37\lib\site-packages\django\utils\functional.py", line 257, in inner
    return func(self._wrapped, *args)
AttributeError: 'AnonymousUser' object has no attribute '_meta'
[05/Jun/2019 21:25:28] "POST /auth/anmelden HTTP/1.1" 500 159900

from Django - LoginView: 'AnonymousUser' object has no attribute '_meta'