Loosing session data between requests

Hi, I am facing a strange problem. I have two servers (both Centos 4.5, httpd 2.0.52, php 5.1.6) and in one of them a webmail (uebimiau) looses session data between requests and in another it does not. I've compared php.ini from both with nothing different except for some memory limits they are equal. I've checked both /tmp and the session file sess_xxxxx in one server is empty and in another has the data. Both $_SESSION are ok (I've checked with a var_dump). What could it be? thanks.