It's not a scripting issue, you could create a blank php page with only this one line on it:
<?php session_start(); ?>
and it would generate the same error message.
the problem is that the folder referenced by the session.save_path setting in the php.ini folder does not exist. since it does not exist, it cannot start the session.
Since the session cannot be started, simple scripting practices like setting a session on one page:
$_SESSION['foo'] = "bar";
then writing the session value on a another page:
echo("Foo = ".isset($_SESSION['foo'])?$_SESSION['foo']:"Not Set");
I would hope that a hosting company would recognize that when simple code to set sessions cannot be executed, it is not a scripting issue, but a problem on the servers that they need to help you resolve.
the above script example are valid script that will work if the sessions can be saved.