Django + Gunicorn + Nginx: Bad Request (400) in Debug=True

I'm trying to run my server with Django, nginx and gunicorn. On the development Server, everything went fine. But on the production server, gunicorn always returns a Bad Request (400).

I'm aware that I need to set my ALLOWED_HOSTS variable, and I did. I tried the correct domain, an asterisk, or even setting DEBUG to True. But still, it's always Bad Request (400).

Here is my nginx-config:

server {
    listen 80;

    location /static {
        alias /home/username/sites/sub.domain.example.com/static;
    }

    location / {
        proxy_set_header Host $http_host;
        proxy_pass http://localhost:8000;
    }
}

My wsgi-prod.py file:

import os
os.environ.setdefault("DJANGO_SETTINGS_MODULE", "app.settings_prod")

from django.core.wsgi import get_wsgi_application
application = get_wsgi_application()

The settings_prod.py file (shortened):

DEBUG = False
ALLOWED_HOSTS=["*"]

I start gunicorn the following way (with virtualenv):

gunicorn --bind 127.0.0.1:8000 app.wsgi_prod:application

When I start the server with manage.py runserver --settings=app.settings_prod , the site is accessible. gunicorn's error log shows nothing, and the access log only shows the 400. The static content does work.