502 bad gateway after updates (Debian, NGINX, Let’s Encrypt to ACMEv2 and CMS) - maybe config or php issue?

Question

Can’t reach the website anymore. Get 502 Bad Gateway error.

What happened:

• Updated Let’s Encrypt to ACMEv2 (at least tried, seems it worked) -> 502 error
• Updated Debian 8 to 10 buster (via 9) -> 502 error
• Updated NGINX 1.14.2 -> 502 error
• Updated the CMS (Kirby from v1 to v3 and back to v1, at least tried) -> 502 error

The website is still not reachable. But other files and folders are reachable (e.g. domain.com/folder-added-manually/index.html

I spent hours testing many solutions. Please advise me. Let me know which info I can provide.

Thank you very much.

var/www/domain/index.php

<?php 

/*

---------------------------------------
Document root of your site
---------------------------------------
this should be identical with the directory 
in which your index.php is located

*/

$root = dirname(__FILE__);



/*

---------------------------------------
Kirby system folder
---------------------------------------

by default this is located inside the root directory
but if you want to share one system folder for 
multiple sites, you can easily change that here
and link to a shared kirby folder somewhere on your
server

*/

$rootKirby = $root . '/kirby';



/*

---------------------------------------
Your site folder
---------------------------------------

Your site folder contains all the site specific files
like templates and snippets. It is located in the root
directory by default, but you can move it if you want.

*/

$rootSite = $root . '/site';



/*

---------------------------------------
Your content folder
---------------------------------------

Your content folder is also located in the root 
directory by default. You can change this here.
It can also be changed later in your site/config.php

*/

$rootContent = $root . '/content';


// Try to load Kirby
if(!file_exists($rootKirby . '/system.php')) {
  die('The Kirby system could not be loaded');  
} 

require_once($rootKirby . '/system.php');

/etc/nginx/nginx.conf

user www-data;
worker_processes auto;
pid /run/nginx.pid;
include /etc/nginx/modules-enabled/*.conf;

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_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/*;
}


#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;
#   }
# 
#   server {
#       listen     localhost:143;
#       protocol   imap;
#       proxy      on;
#   }
#}

/etc/nginx/php.conf

location ~ [^/]\.php(/|$) {
    fastcgi_split_path_info ^(.+?\.php)(/.*)$;
    if (!-f $document_root$fastcgi_script_name) {
        return 404;
    }

    include fastcgi.conf;
    fastcgi_index index.php;
    fastcgi_pass unix:/run/hhvm/hhvm.sock;
}

etc/nginx/domain-common.conf (which is in /etc/nginx/sites-enabled/domain.com as 'include domain-common.conf;')

root /var/www/domain;

index index.php index.html;

include php.conf;
include kirby.conf;