CodeIgniter 404 Page Not Found, but why?

The cause of the problem was that the server was running PHP using FastCGI.

After changing the config.php to

$config['uri_protocol'] = "REQUEST_URI";

everything worked.

You could try one of two things or a combination of both.

1. Be sure that your controller's name starts with a capital letter. eg "Mycontroller.php"
2. If you have not made any changes to your route, for some strange reason, you might have to include capital letters in your url. e.g if your controller is 'Mycontroller.php' with a function named 'testfunc' inside it, then your url will look like this: "http://www.yourdomain/index.php/Mycontroller/testfunc". Note the capital letter. (I'm assuming you haven't added the htaccess file to remove the 'index.php' part. If you have, just remove it from the url.)

I hope this helps someone