It looks like session variables aren't being stored properly on the server. Contact your hosting provider and they should be able to fix this. Usually it is because the value for session_save_path in the php.ini file is set to a folder that doesn't exist or doesn't have the proper privileges.
To test you should be able to go to this page:
then go to this page:
If sessions were being stored properly you would see a session variable on the second page and it's value. Show those pages to your hosting company if they need proof that something is wrong with storing session variables in general.