We try another way to specify the backend to the login function. Even though the original code mentions it is possible : https://docs.djangoproject.com/en/2.2/_modules/django/contrib/auth/#login

app/authentification/utils.py

@@ -16,8 +16,8 @@ def activate(request, uidb64, token):
     if user is not None and account_activation_token.check_token(user, token):
         user.is_active = True
         user.save()
-        login(request=request, user=user,
-              backend='django.contrib.auth.backends.ModelBackend')
+        user.backend = 'django.contrib.auth.backends.ModelBackend'
+        login(request=request, user=user,)
         return redirect('projects')
     else:
         return render(request, 'validate_mail_address_invalid.html')