Client-Side Validation simply does not work on php page

I have a form with file field, text, 2 dropdowns, a set of radio buttons and a text area field.
I started with just validating the text field, nothing else, yet.
the form tag is:
<form action="file_upload_control.php" method="post" enctype="multipart/form-data" name="Auftragsdaten" id="Auftragsdaten" onSubmit="WAValidateNM(document.forms[0].anzahl,'- Invalid number format',1,100,0,'','','.,',document.forms[0].anzahl,1,true)">

But the effect of not inserting any number, is, it jumps to the next page. No validation, no error message, nothing.

I recently updated to VA Toolkit ver. 2.3.5

If I apply DW built-in Form validation behaviour, validation works as expected.

I purchased DataAssists Validation Toolkit in order to use more sophisticated validation. Now it turns out it does even less than DW built-in functionalities.
Any idea?
PS.: I also use an insert record form and DA's file upload, here's part of the code:
// WA_UploadResult1 Params Start
$WA_UploadResult1_Params = array();
// WA_UploadResult1_1 Start
$WA_UploadResult1_Params["WA_UploadResult1_1"] = array(
'UploadFolder' => "../dyn/",
'FileName' => "[FileName]",
'DefaultFileName' => "",
'ResizeType' => "0",
'ResizeWidth' => "0",
'ResizeHeight' => "0",
'ResizeFillColor' => "" );
// WA_UploadResult1_1 End
// WA_UploadResult1 Params End
?><?php
WA_DFP_SetupUploadStatusStruct("WA_UploadResult1");
if(isset($_POST["upload"])){
WA_DFP_UploadFiles("WA_UploadResult1", "dateiraufladen", "0", "", "false", $WA_UploadResult1_Params);
}
?>
<?php
// WA Application Builder Insert
if (isset($_POST["upload"])) // Trigger
{
$WA_connection = $myConnection;
$WA_table = "myTable";
$WA_sessionName = "bst_bsID";
$WA_redirectURL = "";
$WA_keepQueryString = true;
$WA_indexField = "bsID";
.....
?>

I am not sure if this has anything to do with client-side validation, but I thought it might be useful to mention.