There is no post being made to the goto page, so the form information would need to be stored in a session:
<?php
$_SESSION['fieldset_group_Nachricht'] = isset($_POST["fieldset_group_Nachricht"]))?$_POST["fieldset_group_Nachricht"]:"";
?>
you could then echo the session on the goto page
<?php echo((isset($_SESSION["fieldset_group_Nachricht"]))?$_SESSION["fieldset_group_Nachricht"]:"") ?>