This sounds like you have incompatible versions of some libraries installed.
Can you tell us what versions you have installed?
Specifically django and django-allauth?
Thanks for the hint, this caused the issue:
python-django-allauth/stretch-backports,now 0.35.0-1~bpo9+2 all
python-django/stable,now 1:1.10.7-2 all [installed,automatic]
With python-django from backports the issue does not appear.
Also when posting django settings be careful because it contains secrets
that you should change now:
* Mailman credentials
* Database credentials
Oh, I just cared of the database credentials while pasting it, changed.
What I find a little strange is that all your middleware classes are
commented out in the settings you sent.
That doesn't look right...
This was just a test if it would fix something.