On multiple ExpressionEngine installs I have noticed that there is a problem logging in when the following query string parameters are omitted:
admin.php alone (or its renamed aequivalent), I get the following error:
Unable to load your default controller. Please make sure the controller specified in your Routes.php file is valid.
The EE installations affected are set up according to the user guide (but have index.php removed, if that may contribute to the problem).