Could you trim down and attach the generated HTML.  That should explain what's going on.

This is the script at the bottom of my ASPX page when I don't have the "dummy" collapsible panel in there.

<script type="text/xml-script">
<page xmlns:script="http://schemas.microsoft.com/xml-script/2005">
  <components>
    <pageRequestManager id="_PageRequestManager" updatePanelIDs="OneMap1$genericUpdatePanel,OneMap1$zoomSliderUpdatePanel,OneMap1$mapUpdatePanel,OneMap1$featureUpdatePanel,OneMap1$layerLegendUpdatePanel,OneMap1$selectionModeUpdatePanel" asyncPostbackControlIDs="OneMap1_zoomSlider,OneMap1_mapImage" scriptManagerID="OneMap1$scriptManager1" form="form1" />
    <control id="OneMap1_updateProgress" visibilityMode="Collapse">
      <bindings>
        <binding dataContext="_PageRequestManager" dataPath="inPostBack" property="visible" />
      </bindings>
    </control>
  </components>
</page></script>
<script type="text/javascript">
</script>

Here's what the same block looks like with the dummy control added:

<script type="text/xml-script">
<page xmlns:script="http://schemas.microsoft.com/xml-script/2005" xmlns:atlascontrolextender="atlascontrolextender" xmlns:atlascontroltoolkit="atlascontroltoolkit">
  <references>
    <add src="/Test Website/WebResource.axd?d=yTWt3sRX3jLRpsx508_l9xRUXKPeRks1cCBnfqCuf1Zuge5_Y4pPx8uy_GT39DfE1LJnyVEmTKa8URzdjMN8i2f_t52LkJYXwJswRUAvFtdt8D-kph1phXNVVYa6-g8F0&amp;t=632799165960000000" />
    <add src="/Test Website/WebResource.axd?d=DstUA6Q6sJYQ8KU4ZwTaqueea3Vp0kvhueuAypjTVM0yX7xQEFHaeXhBO83GEK_WLeURKwT7c4LAw4cWvtBA-ipdFjhOtR1qXJkkUKuwhsI1&amp;amp;t=632823506800000000" />
    <add src="/Test Website/WebResource.axd?d=DO0ZylcKemD5L-XDrGPlfA3-kCYQytodToH5QNnj3t9kTpaqX4eIyZosl_aqHcP77D4gQr87IqMjrNQl4uVLNeqJP69pbxwO05J6k4wlaDnwlsfBxH4c4dO1fntOXX6Q0&amp;amp;t=632823513440000000" />
  </references>
  <components>
    <pageRequestManager id="_PageRequestManager" updatePanelIDs="OneMap1$genericUpdatePanel,OneMap1$zoomSliderUpdatePanel,OneMap1$mapUpdatePanel,OneMap1$featureUpdatePanel,OneMap1$layerLegendUpdatePanel,OneMap1$selectionModeUpdatePanel" asyncPostbackControlIDs="OneMap1_zoomSlider,OneMap1_mapImage" scriptManagerID="OneMap1$scriptManager1" form="form1" />
    <control id="OneMap1_updateProgress" visibilityMode="Collapse">
      <bindings>
        <binding dataContext="_PageRequestManager" dataPath="inPostBack" property="visible" />
      </bindings>
    </control>
  </components>
</page></script>
<script type="text/javascript">

How can I figure out what those included resources are so I can embed them myself?

I was also getting this Error "Assertion Failed:atlascontrolextender.......

One Quick and dirty solution for this error is drag and drop a CollapsePanelExtender on the screen.

  <atlasToolkit:CollapsiblePanelExtender ID="CollapsiblePanelExtender1test" runat="server">
        </atlasToolkit:CollapsiblePanelExtender>

it worked for me.

Yes, following prakis's recommendation often solves the problem in cases like this.

David Anson:

Yes, following prakis's recommendation often solves the problem in cases like this.

It does as a quick and dirty, and that's what I've been doing.  It's not the best way though and I'm still looking for that solution.

So far, it's the best we know of as well.  If a "bester" solution comes along, we're happy to pass it along!