Multiple Set-Cookie in header? Session regenerating maybe?

Code:

$config['sess_driver']          = 'database';
$config['sess_cookie_name']        = 'ci_session';
$config['sess_use_database']    = TRUE;
$config['sess_save_path']        = 'Ci_sessions';

Now you've removed one that you need (expiration) and you still have one unused (use_database).