Add 'frameBorder="0"' to line 360 as in
<iframe src="vindditnuphpversion.php" width="80%" height="800px" id="formulier" style="margin-top:10px; margin-right:10%; margin-left:10%; margin-bottom:0%; margin-top:100px; width:80%; display:block; float:left;" frameBorder="0"> </iframe>
Note the uppercase letter 'B'.
Laat de leeuw niet in z'n hempie staan.