Keith,
It has nothing to do with web.config file.
Please try the following code and get back to me.
<%@ Page language="c#"%>
<%@ Register Namespace="CuteWebUI" Assembly="CuteWebUI.AjaxUploader" TagPrefix="CuteWebUI" %>
<html>
<head>
</head>
<body>
<form id="Form1" method="post" runat="server">
<CuteWebUI:Uploader runat="server" ID="Uploader1">
</CuteWebUI:Uploader>
</form>
</body>
</html>
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