Sessions different on FCGI or mod_php?

I have a cluster of webservers running on apache-php (with prefork). I'm planning to move from prefork to worker (fcgi). The problem is that I cannot move all servers at once so for a short period of times (matter of days) few of them will be on fcgi and some on modphp. I tested today if the sessions are going to work between these two kind of servers (a test server with fcgi and a real with mod_php) and looks like not, I'm getting logged out when my proxy (basic nginx) is redirecting me from a ws with fcgi to one with mod_php. The application that is hosted on these servers is a magento with few modifications (nothing on the core). If I take the proxy out, everything works perfect, I can change from a node to another and the session is the same.

Is there anything in nginx that can cause this? Below is a part of the config of nginx:

 proxy_ignore_headers "Cache-Control" "Expires";

    proxy_cache_valid  200 302 60m;

    proxy_redirect     off;
    proxy_set_header   Host             $host;
    proxy_set_header   X-Real-IP        $remote_addr;
    proxy_set_header   X-Forwarded-For  $proxy_add_x_forwarded_for;
    proxy_max_temp_file_size 0;

    client_max_body_size       100m;
    client_body_buffer_size    128k;

    proxy_connect_timeout      600000;
    proxy_send_timeout         600000;
    proxy_read_timeout         600000;

    proxy_buffer_size          4k;
    proxy_buffers              4 32k;
    proxy_busy_buffers_size    64k;
    proxy_temp_file_write_size 64k;
    proxy_cache_use_stale  error timeout invalid_header updating http_500 http_502 http_503 http_504;