This error is caused by executing javascript that in anyway changes an element on the page that has not completed loading. The greater issue is, that in this case that element/javascript is in the autogenerated code of the .NET menu. What I have done to fix this issue is to enclose my menu in a div. Give that div an ID and set it display:none. Then in your onload code call a function to set the display of the menu to inline or block (whatever your needs are).
For example:
<body onload="f_onload();">
<div id="divMenu1" style="display: none;">
<asp:Menu ID="Menu1" runat="server" Orientation="Horizontal"></asp:Menu>
</div>
<script type="text/javascript" defer="defer">
function f_onload() {
var divMenu1=document.getElementById("divMenu1");
var divMenu2=document.getElementById("divMenu2");
divMenu1.style.display='inline';
divMenu2.style.display='inline';
}
</script>
</body>
This way, when your user goes to the site, the menu isn't displayed until after the page is finished loading. It is not displayed for only a moment, but that's the moment in which this error occurs.