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

You should never set the permissions for anything to 777 on a web server. There's a detailed answer to why this is the case (and what you should do instead) here: http://serverfault.com/questions/357108/...-webserver

Generally, your server should already have a session path to which the web server has appropriate permissions, so it's just a matter of setting the sess_save_path value to the correct absolute path for your server. If you don't wish to use the default path for your OS, then you should follow the instructions above to configure a directory with the appropriate permissions.

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide