I have the same problem, the menu control is in AJAX Update Panel. Everytime I hover on the menu item, it gave 0.cells error. Below is more info about the error. I put 
where the code got stuck. Thanks !
Call Stack:
Menu_HideItems
Menu_setRoot
Menu_Expand
Menu_HoverStatic
JScript_Form1 anonumous function
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Code:
function Menu_HideItems(items) {
if (document.body.__oldOnClick) {
document.body.onclick = document.body.__oldOnClick;
document.body.__oldOnClick = null;
}
Menu_ClearInterval();
if (!items || ((typeof(items.tagName) == "undefined") && (items instanceof Event))) {
items = __rootMenuItem;
}
var table = items;
if ((typeof(table) == "undefined") || (table == null) || !table.tagName || (table.tagName.toLowerCase() != "table")) {
table = WebForm_GetElementByTagName(table, "TABLE");
}
if ((typeof(table) == "undefined") || (table == null) || !table.tagName || (table.tagName.toLowerCase() != "table")) {
return;
}
var rows = table.rows ? table.rows : table.firstChild.rows;
var isVertical = false;
for (var r = 0; r < rows.length; r++) {
if (rows[r].id) {
isVertical = true;
break;
}
}
var i, child, nextLevel;
if (isVertical) {
for(i = 0; i < rows.length; i++) {
if (rows[i].id) {
child = WebForm_GetElementById(rows[i].id + "Items");
if (child) {
Menu_HideItems(child);
}
}
else if (rows[i].cells[0]) {
nextLevel = WebForm_GetElementByTagName(rows[i].cells[0], "TABLE");
if (nextLevel) {
Menu_HideItems(nextLevel);
}
}
}
}
else {
for(i = 0; i < rows[0].cells.length; i++) {
if (rows[0].cells[i].id) {
child = WebForm_GetElementById(rows[0].cells[i].id + "Items");
if (child) {
Menu_HideItems(child);
}
}
else {
nextLevel = WebForm_GetElementByTagName(rows[0].cells[i], "TABLE");
if (nextLevel) {
Menu_HideItems(rows[0].cells[i].firstChild);
}
}
}
}
if (items && items.id) {
PopOut_Hide(items.id);
}
}