Proxy returns 504 then blocks next connections

[R1CH]

Does your script use sessions? You didn't specify with language it uses, but
several popular languages such as PHP will lock a session file until the
request is complete, preventing multiple requests while the first request
continues to run. Mechanisms similar to session_write_close() in PHP can
work around this, but it sounds like this script should run as a background
process rather than as a web request.

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,240766,240771#msg-240771