After update, problem with routing

Hi,

I've upgraded from CI2.2.3 to CI3.0.6.

Function _fetch_uri_string() was deleted from URI.php and part of its functionality was moved to constructor. Since I set up some defined constants in this function, they were no longer available. Thats why I had to add _fetch_uri_string() to MY_URI.php (actually, I used my old file and function in it) and again call this function from Router.php like in older version of it.

So far so good. Function triggers and defined constants are being available.

BUT, I started getting this error:

An Error Was Encountered
Unable to load the requested file: login.php

I know that file is there and path that is called is localhost/login/
It works with previous version of CI, but stopped with this one, having file structure left intact..

How can I debug this problem?