[Hans]

I Think you forget to run ISPConfig in suPHP mode, because you can not use php_admin_flag directives like: php_admin_flag engine on

To run ISPConfig in suPHP mode:

Edit /home/admispconfig/ispconfig/lib/config.inc.php find the line:

$go_info["server"]["apache2_php"] = 'both';

and change to:

$go_info["server"]["apache2_php"] = 'suphp';

Restart ispConfig afterwards with:

/etc/init.d/ispconfig_server restart

Excuse me if i am wrong, but i think this is the reason.