Nginx 502 Bad Gateway simple Rails App in CentOS 7.x

In CentOS 7.x I’ve installed a Rails app which I can access through [ip]:8080. It works. But when I set the domain name as a server_name (instead of localhost) I get a 502 Bad Gateway problem.

I already restarted nginx. And the application is running with unicorn_rails. My rails log shows nothing. PD: This app is from this tutorial.

/etc/nginx/conf.d/default.conf

upstream app {
    # Path to Unicorn SOCK file, as defined previously
    server unix:/tmp/unicorn.myapp.sock fail_timeout=0;
}

server {

    listen 80;
    server_name mydomain.com www.mydomain.com;

    # Application root, as defined previously
    root /var/www/my_app;

    try_files $uri/index.html $uri @app;

    location @app {
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $http_host;
        proxy_redirect off;
        proxy_pass http://app;
    }

    error_page 500 502 503 504 /500.html;
    client_max_body_size 4G;
    keepalive_timeout 10;
}