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length is null or not an object


I have a SharePoint 2010 Visual Web part, where I am using AjaxControlToolKit (version 3.0.30930.28736) Tab Container. But my Tab Container is inside an update panel. Everything was working fine, until now. In my Development Server, I installed the Cumulative Updates for October 2011 for both SharePoint Foundation and SharePoint Server. After this, I ran the PSConfigUI.exe and restarted the IIS, server and so on. But after all this, when I go to my web part page, where this Tab web part is there, it loads fine. But when I click on the 2nd tab, it shows an error message (JavaScript) saying "'length' is null or not an object." I have an UpdateProgress for TabIndexChanged event and the UpdateProgress keeps showing for ever, because of this javascript error. Any help would be really appreciated.