It could be an issue with session configuration on your live site. Try creating two pages. The first one with:
<?php
@session_start();
$_SESSION["test"] = "working";
?>
The second page put:
<?php
@session_start();
var_dump($_SESSION["test"]);
?>
Then browse the first page followed by the second. See if the session variable value displays "working". If not, then the server is mis-configured. Most likely the session_save_path is pointing to a non-existent directory or a directory without write permissions.