Message: unlink(/tmp/ci_sessionaa51644b4360c8f6957d2c733): Operation not permitted

set permission folder 777 for $config['sess_save_path'] folder(config.php). If you don't define, fallback to default value in php.ini. Alternative, create folder to store session in your client folder and set $config['sess_save_path'] = APPPATH.'sessionfoldername'.

@InsiteFX, you can set $application_folder = 'path/to/application/folder' in index.php

Keep calm.