upstream sent invalid status "-1 Copy failed" while reading response header from upstream

[NaZz]

I don't understand this "upstream" "upstream" "upstream backend server".
The upstream is served by fastcgi - fpm.sock on my own server. How do I
attend this problem?
Where do I dig exactly?

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,254279,254412#msg-254412