502 Bad Gateway nginx when making an upgrade (from v11 to v12)

I could access to NC11 on my web browser using the following address https://DOMAIN:8080/. Then, I upgraded the version of NC11 into NC12. I checked if everything was fine doing a sudo -u nginx php occ -V and gave me Nextcloud version 12.0.13. Seems like Nextcloud is ok. The system I’m using is CentOS 7.
Here is my nginx.conf :

# For more information on configuration, see:
#   * Official English Documentation: http://nginx.org/en/docs/
#   * Official Russian Documentation: http://nginx.org/ru/docs/

user nginx;
worker_processes auto;
error_log /var/log/nginx/error.log;
pid /run/nginx.pid;

# Load dynamic modules. See /usr/share/nginx/README.dynamic.
include /usr/share/nginx/modules/*.conf;

events {
    worker_connections 1024;
}

http {
    log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                      '$status $body_bytes_sent "$http_referer" '
                      '"$http_user_agent" "$http_x_forwarded_for"';

    access_log  /var/log/nginx/access.log  main;

    sendfile            on;
    tcp_nopush          on;
    tcp_nodelay         on;
    keepalive_timeout   65;
    types_hash_max_size 2048;

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

    client_max_body_size 2M;

    # Load modular configuration files from the /etc/nginx/conf.d directory.
    # See http://nginx.org/en/docs/ngx_core_module.html#include
    # for more information.
    include /etc/nginx/conf.d/*.conf;

#    server {
#        listen       80 default_server;
#        listen       [::]:80 default_server;
#        server_name  _;
#        root         /usr/share/nginx/html;
#
#        # Load configuration files for the default server block.
#        include /etc/nginx/default.d/*.conf;
#
#        location / {
#        }
#
#        error_page 404 /404.html;
#            location = /40x.html {
#        }
#
#        error_page 500 502 503 504 /50x.html;
#            location = /50x.html {
#        }
#    }

# Settings for a TLS enabled server.
#
#    server {
#        listen       443 ssl http2 default_server;
#        listen       [::]:443 ssl http2 default_server;
#        server_name  _;
#        root         /usr/share/nginx/html;
#
#        ssl_certificate "/etc/pki/nginx/server.crt";
#        ssl_certificate_key "/etc/pki/nginx/private/server.key";
#        ssl_session_cache shared:SSL:1m;
#        ssl_session_timeout  10m;
#        ssl_ciphers HIGH:!aNULL:!MD5;
#        ssl_prefer_server_ciphers on;
#
#        # Load configuration files for the default server block.
#        include /etc/nginx/default.d/*.conf;
#
#        location / {
#        }
#
#        error_page 404 /404.html;
#            location = /40x.html {
#        }
#
#        error_page 500 502 503 504 /50x.html;
#            location = /50x.html {
#        }
#    }

}

Error I get from /var/log/nginx/error.log

2019/02/01 16:06:30 [error] 6255#0: *35 connect() failed (111: Connection refused) while connecting to upstream, client: CLIENT_IP, server: DOMAINNAME, request: "GET / HTTP/1.1", upstream: "fastcgi://127.0.0.1:9000", host: "DOMAIN:8080"
2019/02/01 16:06:30 [error] 6255#0: *35 connect() failed (111: Connection refused) while connecting to upstream, client: CLIENT_IP, server: DOMAINNAME, request: "GET /favicon.ico HTTP/1.1", upstream: "fastcgi://127.0.0.1:9000", host: "DOMAIN:8080", referrer: "https://DOMAIN:8080/"