cookie based proxy redirect

Hi, I'm using nginx in a reverse proxy setup with 2 apache 2.4 backends, I've configured a cookie based proxy_pass setup as shown in http://dgtool.blogspot.com/2013/02/nginx-as-sticky-balancer-for-ha-using.html , however php header redirects seem to die (or the browser doesn't like) when nginx goes to redirect them internally with the '502 bad gateway' which should forward it to the upstream pool.

All other urls work fine, but when a request comes in with a header based redirect, something is getting stuck on the 502 bad gateway, the browser just stops with a blank page. Not sure if I can add something to nginx to fix...

Thanks