Why does 500 NGINX error sometimes crash?

Question

Nginx

There is a configuration for wordpress:

server {
  listen 80;
  server_name sitename.ru *.sitename.ru;
  return 301 https://$host$request_uri;
}

server {
  listen 443 ssl;
  server_name sitename.ru *.sitename.ru;
  ssl_certificate /etc/letsencrypt/live/sitename.ru/fullchain.pem; # managed by Certbot
  ssl_certificate_key /etc/letsencrypt/live/sitename.ru/privkey.pem; # managed by Certbot
  ssl_protocols       TLSv1 TLSv1.1 TLSv1.2;
  ssl_ciphers         HIGH:!aNULL:!MD5;

  root /var/www/html/sitename.ru; # путь к WP
  index index.php;

        gzip on; # включаем сжатие gzip
        gzip_disable "msie6";
        gzip_types text/plain text/css application/json application/x-javascript text/xml application/xml application/xml+rss text/javascript application/javascript;

        location ~ /\. {
                deny all; # запрет для скрытых файлов
        }

        location ~* /(?:uploads|files)/.*\.php$ {
                deny all; # запрет для загруженных скриптов
        }

        location ~* ^.+\.(ogg|ogv|svg|svgz|eot|otf|woff|mp4|ttf|rss|atom|jpg|jpeg|gif|png|ico|zip|tgz|gz|rar|bz2|doc|xls|exe|ppt|tar|mid|midi|wav|bmp|rtf)$ {
                access_log off;
                log_not_found off;
                expires max; # кеширование статики
        }

        location / {
                try_files $uri $uri/ /index.php?$args; # permalinks
        }

        location ~ \.php$ {
        fastcgi_pass unix:/var/run/php/php7.2-fpm.sock;
        fastcgi_index index.php;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        include fastcgi_params;
        }
}

And FAST CGI options:

fastcgi_param  QUERY_STRING       $query_string;
fastcgi_param  REQUEST_METHOD     $request_method;
fastcgi_param  CONTENT_TYPE       $content_type;
fastcgi_param  CONTENT_LENGTH     $content_length;
fastcgi_param  SCRIPT_NAME        $fastcgi_script_name;
fastcgi_param  REQUEST_URI        $request_uri;
fastcgi_param  DOCUMENT_URI       $document_uri;
fastcgi_param  DOCUMENT_ROOT      $document_root;
fastcgi_param  SERVER_PROTOCOL    $server_protocol;
fastcgi_param  HTTPS              $https if_not_empty;
fastcgi_param  GATEWAY_INTERFACE  CGI/1.1;
fastcgi_param  SERVER_SOFTWARE    nginx/$nginx_version;
fastcgi_param  REMOTE_ADDR        $remote_addr;
fastcgi_param  REMOTE_PORT        $remote_port;
fastcgi_param  SERVER_ADDR        $server_addr;
fastcgi_param  SERVER_PORT        $server_port;
fastcgi_param  SERVER_NAME        $server_name;
fastcgi_param  SCRIPT_FILENAME    $document_root$fastcgi_script_name;
fastcgi_param  REDIRECT_STATUS    200;

500 error crashes in random downloads of the site. I don't understand why. Maybe something in my config is wrong?
The NGINX config is basically like this:

user www-data;
worker_processes 2;
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;
  
  client_max_body_size 999m;	

  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/*;
}

Answer 1

First you need to study the server logs. it is very likely that the cause of the error is already written there