[vbmechanic]

Try these:

<div><label for="cb_onfrontpage"><input type="checkbox" name="onfrontpage" value="1" id="cb_onfrontpage" tabindex="1" $checked[onfrontpage] />Auf der Startseite?</label></div>

and

// ############################### start do set on frontpage ###############################
 if ($_POST['onfrontpage'] == 1)
 {
 	$DB_site->query("UPDATE " . TABLE_PREFIX . "thread SET onfrontpage = 1 WHERE threadid = $threadid");
 
 	$_REQUEST['forceredirect'] = 1;
 	$url = "showthread.php?$session[sessionurl]t=$threadid";
 	eval(print_standard_redirect('redirect_openclose'));
 
 }