ajaxToolkit:AjaxFileUpload Issues

Hi,

I have a file upload control that has been working fine until I made some unrelated (in theory) changes over the last couple of weeks and now the file upload doesn't work.

https://imgur.com/a/DAGRN

As a test, I created a new file upload:

<ajaxToolkit:AjaxFileUpload ID="AjaxFileUpload1" runat="server" />

But as soon as I apply client events, it doesn't render (see above):

OnClientUploadStart="UploadStart" OnClientUploadComplete="UploadComplete"

I've compared the output and the two versions are identical, apart from the initialisation function that is automatically created:

// PREVIOUS BUILD (WORKING) ...<script type="text/javascript">
//<![CDATA[
Sys.Application.add_init(function() {$create(Sys.Extended.UI.AjaxFileUpload.Control, {"allowedFileTypes":"mp3","clearFileListAfterUpload":true,"clientStateField":$get("cphPageContent_ModalAudioUpload_AjaxFileUpload_ClientState"),"contextKey":"cphPageContent_ModalAudioUpload_AjaxFileUpload","maximumNumberOfFiles":1,"postBackUrl":"/my_page?id=12345","serverPollingSupport":true}, {"uploadComplete":UploadComplete,"uploadStart":UploadStart}, null, $get("cphPageContent_ModalAudioUpload_AjaxFileUpload"));
});
//]]></script></form>

// CURRENT BUILD (NOT WORKING) ...<script type="text/javascript">
//<![CDATA[
Sys.Application.add_init(function() {$create(Sys.Extended.UI.AjaxFileUpload.Control, {"allowedFileTypes":"mp3","clearFileListAfterUpload":true,"clientStateField":$get("cphPageContent_ModalAudioUpload_AjaxFileUpload_ClientState"),"contextKey":"{DA8BEDC8-B952-4d5d-8CC2-59FE922E2923}","enabled":true,"maximumNumberOfFiles":1,"postBackUrl":"/my_page?id=12345","serverPollingSupport":true,"uploadHandlerPath":"/AjaxFileUploadHandler.axd"}, {"uploadComplete":UploadComplete,"uploadStart":UploadStart}, null, $get("cphPageContent_ModalAudioUpload_AjaxFileUpload"));
});
//]]></script></form>

Anyone know what's happened and how to fix?