我在错误日志中收到一个nginx错误，如下所示：

[error] 4257#0: *3470 upstream timed out (110: Connection timed out) while reading response header from upstream, client 120.X.X.X

这是一个运行AWS并使用nginx作为反向代理的WordPress Web服务器。

user                 nginx nginx;
worker_processes     2;
worker_rlimit_nofile 10240;

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

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

events {
worker_connections  8192;
use epoll;
accept_mutex_delay 100ms;
}

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

log_format main     '$remote_addr $server_name $remote_user [$time_local] "$request" '
                    '$status $body_bytes_sent "$http_referer" '
                    '"$http_user_agent" "$http_x_forwarded_for" $request_time';
log_format backend  '$remote_addr $server_name $remote_user [$time_local] "$request" '
                    '$status $body_bytes_sent "$http_referer" '
                    '"$http_user_agent" "$http_x_forwarded_for" $request_time';
log_format proxylog '$remote_addr $server_name $remote_user [$time_local] "$request" '
                    '$status $body_bytes_sent "$http_referer" '
                    '"$http_user_agent" "$http_x_forwarded_for" $request_time '
                    '"$http_proxy"';

access_log  /var/log/nginx/access.log  main;
access_log  /var/log/nginx/badactor.log proxylog if=$http_proxy;

server_name_in_redirect off;
server_tokens      off;

sendfile           on;
open_file_cache    max=100 inactive=20s;
tcp_nopush         on;

keepalive_timeout  5;

client_max_body_size    4M;
client_body_buffer_size 256k;

if_modified_since before;

gzip              on;
gzip_http_version 1.0;
gzip_vary         on;
gzip_comp_level   6;
gzip_types        text/plain
                  text/xml
                  text/css
                  text/javascript
                  application/xhtml+xml
                  application/xml
                  application/rss+xml
                  application/atom_xml
                  application/javascript
                  application/x-javascript
                  application/x-httpd-php;
gzip_disable      "MSIE [1-6]\.";

# proxy cache
proxy_cache_path  /var/cache/nginx/proxy_cache levels=1:2
                  keys_zone=czone:32m max_size=256m inactive=1440m;
proxy_temp_path   /var/cache/nginx/proxy_temp;
proxy_cache_key   "$scheme://$host$request_uri";
proxy_set_header  Host               $host;
proxy_set_header  X-Real-IP          $remote_addr;
proxy_set_header  Remote-Addr        $remote_addr;
proxy_set_header  X-Forwarded-Host   $host;
proxy_set_header  X-Forwarded-Server $host;
proxy_set_header  X-Forwarded-For    $proxy_add_x_forwarded_for;
proxy_set_header  X-Forwarded-Proto  $scheme;
proxy_set_header  X-UA-Detect        $mobile;
proxy_set_header  Accept-Encoding    "";
proxy_set_header  Proxy              "";
proxy_hide_header X-Pingback;
proxy_hide_header Link;
proxy_hide_header ETag;
proxy_connect_timeout 5;
proxy_send_timeout 10;
proxy_read_timeout 90;
proxy_cache_use_stale timeout invalid_header http_500 http_502 http_503 http_504;
proxy_cache_lock on;
proxy_cache_lock_timeout 5s;
proxy_buffers 8 32k;
proxy_buffer_size 64k;

# fastcgi cache
#fastcgi_cache_path /var/cache/nginx/fastcgi_cache levels=1:2
#                   keys_zone=fastcgizone:32m inactive=60m;
#fastcgi_cache_key "$scheme$request_method$host$request_uri";
#fastcgi_cache_use_stale error timeout invalid_header http_500;

#limit_req_zone $request_method zone=method:1m rate=250r/s;

upstream backend {
    server unix:/var/run/nginx-backend.sock;
}

upstream phpfpm {
    server unix:/var/run/php-fpm.sock;
}

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

在客户端，收到一个http 502（坏网关）。可能是什么错误？