After some ubuntu 16.04 upgrades and app code modifications, my Flask on gunicorn and nginx website which previously was working is now giving a 502 bad gateway error.

myapp.conf:

upstream app_server_wsgiapp {
  server localhost:8000 fail_timeout=0;
}

server {
  listen 80;
  server_name www.myserver.com;

  access_log   /var/log/nginx/www.myapp.access.log;
  error_log   /var/log/nginx/www.myapp.error.log info;
  keepalive_timeout    5;

  location /static {
    autoindex on;
    alias /myapp/static;
  }

  location / {
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header Host $http_host;
    proxy_redirect off;
    if (!-f $request_filename) {
      proxy_pass http://app_server_wsgiapp;
      break; 
    }
    client_max_body_size 2097152;
    #to get around upstream sent too big header while reading response header from upstream error
    proxy_buffer_size          128k;
    proxy_buffers              4 256k;
    proxy_busy_buffers_size    256k;
  }

  # this section allows Nginx to reverse proxy for websockets
  location /socket.io {
    proxy_pass http://app_server_wsgiapp/socket.io;
    proxy_redirect off;
    proxy_buffering off;
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection "Upgrade";
  } 
}

/var/log/nginx/myapp.error.log

2017/06/11 06:42:52 [error] 31054#31054: *1 connect() failed (111: Connection refused) while connecting to upstream, client: clientip, server: www.myserver.com, request: "GET / HTTP/1.1", upstream: "http://127.0.0.1:8000/", host: "www.myapp.com"

From my apps log file I note that Flask gets part of the way through starting up, then restarts in a continuous fail/restart loop.

Any ideas how I can go about debugging what could be causing this issue?

Looks like the problem is caused by flask app running error. Could you please try to run the app in interactive way? With same params and environment settings in daemon way and check whether the service can be started correctly?