I newly installed an elastix and when started from adding some extensions I found the extension config page under PBX menu couldn't be opened --- it was blank there but there were something supposed to be there: like add SIP extension and so on. I looked through the /var/www/html/admin/core and the page.extension.php was there but only contain several lines' code. Did I make any mistakes during installation? The elastix I installed is Elastix-1.3-stable-29sep2008.iso. Any ideas will be great appreciated.