This is mostly caused by HTML errors introduced in a module.

My first suspect would be that module with the javascript in the contentpane:

<div class="contentarea_middle" id="mnt">
                                   <div id="dnn_ContentPane" class="ContentPane"><a name="469"></a><span id="dnn_ctr469_ContentPane" class="DNNAlignleft"><!-- Start_Module_469 --><div id="dnn_ctr469_ModuleContent">
    <div id="dnn_ctr469_MLHTML_MLHTML_HtmlHolder" class="Normal"></div>

<div id="dnn_ctr469_MLHTML_MLHTML_lblContent" class="Normal">
    <script language="javascript" type="text/javascript">
function showHide(shID) {
   if (document.getElementById(shID)) {
      if (document.getElementById(shID+'-show').style.display != 'none') {
         document.getElementById(shID+'-show').style.display = 'none';
         document.getElementById(shID).style.display = 'block';
      }
      else {
         document.getElementById(shID+'-show').style.display = 'inline';
         document.getElementById(shID).style.display = 'none';
      }
   }
}
</script>
<p><style type="text/css"> <!--{13548878865200}-P</style></p>
<p>&#160;</p>
</div>

 <!-- End_Module_469 -->
</div></span>
</div>

 The closing span is not supposed to be there it seems?
At least the validator complains about a div not being closed