Login code in controllers\login.php looks to URI segment 3 and 4 for username and password or POST variables username and password. So try "http://localhost/open-audit/index.php/login/login_auth/specify_username/specify_password"
It does look like the response is not correct for a properly authenticated LDAP login for a non-admin user. Seems like line 386
Code:
echo '{"valid": false, "admin": false}';
should be
Code:
echo '{"valid": true, "admin": false}';
And the response header should be 200 not 403.