Gunicorn unable to start after server update ( 502 bad gateway )

Hello, I have updated my old Ubuntu server from version 14 to 16. This also included a Django and Python update. After restarting the server, I got a 502 error from Nginx, which I have discovered is being caused by Gunicorn not being able to start. I am able to start gunicorn manually, and the site works perfectly, by doing:

root@server:/home/django/django_project/django_project# gunicorn -b 127.0.0.1:9000 wsgi

wsgi.py is located at /home/django/django_project/django_project

The problem I am having is that the gunicorn service is not able to start automatically, even if the config (/etc/init/gunicorn.conf) looks correct:

[...]

chdir /home/django
exec gunicorn \
--name=django_project \
--pythonpath=django_project \
--bind=127.0.0.1:9000 \
--config /etc/gunicorn.d/gunicorn.py \
django_project.wsgi:application 

Any ideas of why the service is not starting by itself anymore?

service gunicorn restart

does not work either.