Some times getting 500 Internal error or show network error on firebug in Web browser. When checked on NGINX error log using tail commnad,

#  tail -f /var/log/nginx/error_log

2015/04/01 17:41:15 [crit] 9467#0: *2 open() "/tmp/nginx_client/0012068091"
failed (28: No space left on device), client:, server:,    request: "POST /index.php HTTP/1.0", host: ""

We need to clear  give space "tmp" directory, ( * ) symbol spcified that current path,

Stop the NGINX service,
#  /etc/init.d/nginx  stop

Check your /tmp partition space and clear  files or directories,
# df  -h

# cd /tmp 

# rm -rf *  

Finally start your NGINX server,
# /etc/init.d/nginx  start