Asp.Net Ajax Toolkit AsyncFileUpload - "The file is attached is invalid" error

I'm trying to use the AsyncFileUpload control from the Asp.net Ajax Control Toolkit (Sept 30, 2009 stable build: 30930) . I've created a demo application and the control works fine. Files upload and all is well.

When I try to use the control in my real application, I'm always receiving an error stating "The file attached is invalid". The AsyncFileUpload control returns this when the file uploaded is null.

To isolate the problem, I created a new master page exactly like the master page in my demo app. I also created an aspx page exactly like the page in my demo app. The upload still fails with a "The file attached is invalid" error.

I also compared the web.config for the real app and the demo app and couldn't identify any differences that should matter.

The code below is the test code in my real app. This code is exactly the same as the functioning code in the demo app with the exception of having different class names and file names. The web.config listed below is from the real app with the appsettings and connectionstrings removed.

At this point, I am completely stumped.

Real App Test Master Page:

<%@ Master Language="C#" AutoEventWireup="true" CodeFile="aaaMasterPage.master.cs" Inherits="L1Admin.aaaMasterPage" %>

<!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 runat="server">
    <title>Untitled Page</title>
    <script type="text/javascript" src="js/jquery-1.3.2.min.js"></script>    
    <asp:ContentPlaceHolder id="head" runat="server">
    </asp:ContentPlaceHolder>
</head>
<body>
    <form id="form1" runat="server">
    <asp:ScriptManager ID="scriptManager" EnablePartialRendering="true" runat="server" AsyncPostBackTimeout="180" />    
    <div>
        <asp:ContentPlaceHolder id