Upgrade 2.0.2 to 3.1.10 session problem on local mamp

Update:

the config is good because I can trick something to get the session working. But, when I logged a new session is created, then I redirect and validate in the redirectController if I'm logged, but then the session_id change.

I've a library Authentification that I autoload. I do in the constructor : $this->CI =& get_instance();

It's like when I redirect, I get a new session and the weird thing is when I redirect for a second time(because where're not logged in) the session is ok.
Thanks for any help