Hi dacoda,
Can you post your web.config code here? So we can check it for you.
And please try the example page below, does it work for you?
<%@ 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 ID="uploader1" runat="server">
</CuteWebUI:UploadAttachments>
</form>
</body>
</html>
Regards,
Ken