On some hostings you can receive this error.
A PHP Error was encountered Severity: Warning Message: unlink(/tmp/ci_session0189a7f1c86eb18fb70afcfedc2d5040e9c23146): Operation not permitted Filename: drivers/Session_files_driver.php
To fix this go to application > config > config.php and edit this line:
$config['sess_save_path'] = BASEPATH . 'cache/';