Nginx + Django + Gunicorn: 502 Bad Gateway upstream prematurely closed connection while reading response header from upstream

Question

I am creating (for the first time) a website. More specifically, the user uploads an image and after some computations the website displays some result.

However, sometimes (not always, sometimes just retrying or changing image it works) during this process I get "502 Bad Gateway nginx/1.10.3 (Ubuntu)"

Looking at the error I get:

[error] 7836#7836: *16 upstream prematurely closed connection while reading response header from upstream, client: 131.175.147.1, server: _, request: "POST / HTTP/1.1", upstream: "http://unix:/home/django/gunicorn.socket:/", host: "165.227.170.231", referrer: "http://165.227.170.231/"

I tried this but it did not help much, while I think this is not my case.

The settings are:

/etc/nginx/sites-available/django :

upstream app_server {
server unix:/home/django/gunicorn.socket fail_timeout=0;
}

server {
listen 80 default_server;
listen [::]:80 default_server ipv6only=on;

root /usr/share/nginx/html;
index index.html index.htm;

client_max_body_size 4G;
server_name _;

keepalive_timeout 5;

# Your Django project's media files - amend as required
location /media  {
    alias /home/django/django_project/django_project/media;
}   

# your Django project's static files - amend as required
location /static {
    alias /home/django/django_project/django_project/static;
}   

# Proxy the static assests for the Django Admin panel
location /static/admin {
   alias /usr/lib/python2.7/dist-packages/django/contrib/admin/static/admin/;
}  

location / {
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $host;
        proxy_redirect off;
        proxy_buffering off;

        proxy_connect_timeout 75s;
        proxy_read_timeout 300s;



        proxy_pass http://app_server;
        }
}

The /etc/nginx/nginx.conf:

user www-data;
worker_processes auto;
pid /run/nginx.pid;

events {
    worker_connections 768;
    # multi_accept on;
}

http {

    ##
    # Basic Settings
    ##

    sendfile on;
    tcp_nopush on;
    tcp_nodelay on;
    keepalive_timeout 65;
    types_hash_max_size 2048;
    # server_tokens off;

    # server_names_hash_bucket_size 64;
    # server_name_in_redirect off;

    include /etc/nginx/mime.types;
    default_type application/octet-stream;

    ##
    # SSL Settings
    ##

    ssl_protocols TLSv1 TLSv1.1 TLSv1.2; # Dropping SSLv3, ref: POODLE
    ssl_prefer_server_ciphers on;

    ##
    # Logging Settings
    ##

    access_log /var/log/nginx/access.log;
    error_log /var/log/nginx/error.log;
   ##
    # Gzip Settings
    ##

    gzip on;
    gzip_disable "msie6";

    # gzip_vary on;
    # gzip_proxied any;
    # gzip_comp_level 6;
    # gzip_buffers 16 8k;
    # gzip_http_version 1.1;
    # gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;

    ##
    # Virtual Host Configs
    ##

    include /etc/nginx/conf.d/*.conf;
    include /etc/nginx/sites-enabled/*;

    proxy_connect_timeout 75s;
    proxy_read_timeout 300s;
  }


  #mail {
  #       # See sample authentication script at:
  #       # http://wiki.nginx.org/ImapAuthenticateWithApachePhpScript
  #
  #       # auth_http localhost/auth.php;
  #       # pop3_capabilities "TOP" "USER";
  #       # imap_capabilities "IMAP4rev1" "UIDPLUS";
  #
  #       server {
  #               listen     localhost:110;
  #               protocol   pop3;
  #               proxy      on;

  #       }
  #}

The /ect/gunicorn.d/gunicorn.py:

"""gunicorn WSGI server configuration."""
from multiprocessing import cpu_count
from os import environ


def max_workers():
    return cpu_count() * 2 + 1

    max_requests = 1000
    worker_class = 'gevent'
    workers = max_workers()
    TIMEOUT=120

And I start gunicorn in this way:

service gunicorn start --timeout 300 

Basically, I tried everywhere to avoid timing out, but still encounter the same problem.