hello

when i update template in vbulletin php forum i got this error in vhost-error_log

2012/09/09 23:13:16 [error] 10313#0: *3877 upstream prematurely closed connection while reading response header from upstream, client: 192.168.1.1, server: site.net, request: "POST /vb/admincp/template.php?do=updatetemplate HTTP/1.1", upstream: "http://66.212.18.72:8081/vb/admincp/template.php?do=updatetemplate", host: "www.site.net", referrer: "http://www.site.net/vb/login.php?do=login"
2012/09/09 23:14:17 [error] 10313#0: *5145 upstream prematurely closed connection while reading response header from upstream, client: 192.168.1.1, server: site.net, request: "POST /vb/admincp/template.php?do=updatetemplate HTTP/1.1", upstream: "http://66.212.18.72:8081/vb/admincp/template.php?do=updatetemplate", host: "www.site.net", referrer: "http://www.site.net/vb/admincp/template.php?s=&do=edit&templateid=2206&group=ad_&searchstring=&expandset=32"
2012/09/09 23:16:02 [error] 10313#0: *6896 upstream prematurely closed connection while reading response header from upstream, client: 192.168.1.1, server: site.net, request: "POST /vb/admincp/template.php?do=updatetemplate HTTP/1.1", upstream: "http://66.212.18.72:8081/vb/admincp/template.php?do=updatetemplate", host: "www.site.net", referrer: "http://www.site.net/vb/admincp/template.php?s=&do=edit&templateid=2206&group=ad_&searchstring=&expandset=32"


so what's the problem

thanks