I have setup a subdomain to proxy my plesk panel, but when saving pages I am getting 502 Bad Gateway error instead of a completion message. I am running CentOS 6.

Here is my vhost.conf configuration for http://plesk.domain.tld/:

RewriteEngine On
RewriteCond %{SERVER_PORT} ^80$
RewriteRule $ https://plesk.domain.tld/ [R,L]

Here is my vhost_ssl.conf configuration for https://plesk.domain.tld/:

SSLProxyEngine On

<Location />
    ProxyPass https://localhost:8443/
    ProxyPassReverse https://localhost:8443/
</Location>

I have more than enough (and I have even checked) RAM, CPU and HDD. There are no spikes. As well, the posted information does save, it just errors when trying to show me a "This information has been saved." green/red block.

Here is the relevent error from /var/log/nginx/error.log (IP/Host Filtered):

2014/05/29 02:42:41 [error] 8046#0: *402 upstream prematurely closed connection while reading response header from upstream, client: 173.238.XX.XX, server: plesk.domain.tld, request: "POST /smb/web/edit HTTP/1.1", upstream: "https://198.100.XX.XX:7081/smb/web/edit", host: "plesk.domain.tld", referrer: "https://plesk.domain.tld/smb/web/edit"