Как победить ошибку: "(104: Connection reset by peer) while reading response header from upstream?

Question

Slash @Slash-Zn

PHP
Nginx

Как победить ошибку: "(104: Connection reset by peer) while reading response header from upstream?

Здравствуйте.
Ubuntu 16, FastCGI (Nginx + PHP-FPM), PHP 7.2.

Со вчерашнего дня сайт периодически выдает 502 ошибку, в лог пишет:

2018/09/02 08:41:18 [error] 7016#7016: *7584 recv() failed (104: Connection reset by peer) while reading response header from upstream, client: xx.xxx.xxx.xxx, server: site.com, request: "GET /page/url/ HTTP/1.1", upstream: "fastcgi://unix:/run/php/php7.2-fpm.sock:", host: "site.com"

Мой конфиг:

user www-data;
worker_processes auto;
worker_rlimit_nofile 65000;
timer_resolution 100ms;
worker_priority -5;

error_log  /var/log/nginx/error.log warn;
pid        /var/run/nginx.pid;

events {
	worker_connections 4096;
	multi_accept on;
	use epoll;
}

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

	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;
	keepalive_timeout 65;

	reset_timedout_connection on;

	include /etc/nginx/conf.d/*.conf;
	include /etc/nginx/vhosts/*/*.conf;

	client_body_buffer_size 8m;
	client_max_body_size    128m;

	open_file_cache max=200000 inactive=20s; 
	open_file_cache_valid 30s; 
	open_file_cache_min_uses 2;
	open_file_cache_errors on;

	server {
		server_name      localhost;
		disable_symlinks if_not_owner;

		include /etc/nginx/vhosts-includes/*.conf;

		location @fallback {
			error_log /dev/null crit;

			proxy_pass http://127.0.0.1:8080;
			proxy_redirect http://127.0.0.1:8080 /;
			proxy_set_header Host $host;
			proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
			proxy_set_header X-Forwarded-Proto $scheme;

			access_log off ;
		}

		listen 80;
		listen [::]:80;
	}
}

И второй конфиг:

server {
	server_name site.com www.site.com;
	charset UTF-8;
	index index.php;
	disable_symlinks if_not_owner from=$root_path;
	include /etc/nginx/vhosts-includes/*.conf;
	include /etc/nginx/vhosts-resources/site.com/*.conf;
	error_log /var/www/httpd-logs/site.com.error.log crit;
	ssi on;
	return 301 https://$host:443$request_uri;
	set $root_path /var/www/www-root/data/www/site.com;
	root $root_path;
	gzip on;
	gzip_comp_level 5;
	gzip_disable "msie6";
	gzip_types text/plain text/css application/json application/x-javascript text/xml application/xml application/xml+rss text/javascript application/javascript;
	error_log /var/www/httpd-logs/site.com.error.log notice;
	location ~* ^.+\.(jpg|jpeg|gif|png|ico|css|js|ico)$ {
		expires 7d;
	}
	location / {
		try_files $uri /index.php?do=$uri;
		location ~ [^/]\.ph(p\d*|tml)$ {
			try_files /does_not_exists @php;
		}
	}
	location @php {
		fastcgi_index index.php;
		fastcgi_param PHP_ADMIN_VALUE "sendmail_path = /usr/sbin/sendmail -t -i -f znleha@yandex.ru";
		fastcgi_pass unix:/run/php/php7.2-fpm.sock;
		fastcgi_split_path_info ^((?U).+\.ph(?:p\d*|tml))(/?.+)$;
		try_files $uri =404;

		fastcgi_buffers 4 512k;
		fastcgi_busy_buffers_size 512k;
		fastcgi_temp_file_write_size 512k;

		include fastcgi_params;
	}
	access_log off;
	listen 77.222.63.224:80 default_server;
	listen [2a02:408:7722:54:77:222:63:224]:80 default_server;
}
server {
	server_name site.com www.site.com;
	ssl_certificate "/var/www/httpd-cert/www-root/site.com_le1.crtca";
	ssl_certificate_key "/var/www/httpd-cert/www-root/site.com_le1.key";
	ssl_ciphers EECDH:+AES256:-3DES:RSA+AES:!NULL:!RC4;
	ssl_prefer_server_ciphers on;
	ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
	add_header Strict-Transport-Security "max-age=31536000;";
	ssl_dhparam /etc/ssl/certs/dhparam4096.pem;
	charset UTF-8;
	index index.php;
	disable_symlinks if_not_owner from=$root_path;
	include /etc/nginx/vhosts-includes/*.conf;
	include /etc/nginx/vhosts-resources/site.com/*.conf;
	error_log /var/www/httpd-logs/site.com.error.log crit;
	ssi on;
	set $root_path /var/www/www-root/data/www/site.com;
	root $root_path;
	gzip on;
	gzip_comp_level 5;
	gzip_disable "msie6";
	gzip_types text/plain text/css application/json application/x-javascript text/xml application/xml application/xml+rss text/javascript application/javascript;
	error_log /var/www/httpd-logs/site.com.error.log notice;
	listen [2a02:408:7722:54:77:222:63:224]:443 ssl default_server;
	listen 77.222.63.224:443 ssl default_server;
	location ~* ^.+\.(jpg|jpeg|gif|png|ico|css|js|ico)$ {
		expires 7d;
	}
	location / {
		try_files $uri /index.php?do=$uri;
		location ~ [^/]\.ph(p\d*|tml)$ {
			try_files /does_not_exists @php;
		}
	}
	location @php {
		fastcgi_index index.php;
		fastcgi_param PHP_ADMIN_VALUE "sendmail_path = /usr/sbin/sendmail -t -i -f znleha@yandex.ru";
		fastcgi_pass unix:/run/php/php7.2-fpm.sock;
		fastcgi_split_path_info ^((?U).+\.ph(?:p\d*|tml))(/?.+)$;
		try_files $uri =404;

		fastcgi_buffers 4 512k;
		fastcgi_busy_buffers_size 512k;
		fastcgi_temp_file_write_size 512k;

		include fastcgi_params;
	}
	access_log off;
}

Ошибка может появиться в любой момент (может и не появиться)... От куда ноги растут, не пойму...
Подскажите, куда копать?
Спасибо.