ok, is session_start() at the top of all the pages?
This error is common if you're sending something to the browser before you call session_start().
I tend to make it the first thing on every page by default, e.g.
<?php session_start(); include('header.php'); include_once('connect.php') echo <<<HTML <html> <head> ..... ?>
Made it the first line in the code, and now get a blank page, basically. Look and see!