404 not found!

I see two problems...

1) The "default_controller" setting is used to define the name of the controller class to use inside a controllers folder; it is not meant to denote a controller and method. Your setting says to look for the "users/login" controller, which doesn't make sense.
This means that "http://localhost/login" could map to the default method of the Login controller or to "users/login" inside a folder "application/controllers/login".

2) If you want a controller "subit_backend", then "appllication/controllers/Subit_backend.php" needs to define a class "Subit_backend" and not a "register" class.