"Unable to locate the specified class: Session.php"...

A file in the Core could be incomplete and or corrupted. Download a fresh CodeIgniter package and overwrite the system directory with the freshly downloaded one.

EDIT

Are you using

PHP Code:

load_class("Session/Session", "core") 


for session loading or are you loading via

PHP Code:

$this->load->library("session") 


Done yesterday, while i'm getting crazy with this problem. My "system" forlder is a fresh downloaded version (CI 3.1.4).
No such way to get the correct loading of the sessions....

About my loading:
Absolutely i'm not calling load_class.
Also, the loading of Session is not by the loader class in the controller. I use CI autoload (autoload.php)

PHP Code:

$autoload['libraries'] = array('session'); 


If i remove this, and i load session by "$this->load->library("session")" no change. Still getting the issue.