If you edit the file "model/admin/webserver.php", remove the delete_oke() function and add the following function, it should work.
public function delete_item($item_id) {
$queries = array(
array("delete from actions where webserver_id=%d", $item_id),
array("delete from host_statistics where webserver_id=%d", $item_id),
array("delete from requests where webserver_id=%d", $item_id),
array("delete from server_statistics where webserver_id=%d", $item_id),
array("delete from webservers where id=%d", $item_id));
return $this->db->transaction($queries);
}