Hi MattW,
Please create an example page by the code below and put it to the root of your site, does it get the same issue?
Can you send me your uploader page url? So I can check it too.
<%@ Page Language="C#" %>
<%@ Register TagPrefix="CuteWebUI" Namespace="CuteWebUI" Assembly="CuteWebUI.AjaxUploader" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
<title>example</title>
</head>
<body>
<form id="form1" runat="server">
<CuteWebUI:UploadAttachments runat="server" ID="uploader1" UploadType="iframe">
</CuteWebUI:UploadAttachments>
</form>
</body>
</html>
Regards,
Ken