I have been developing a project using BlogEngine.NET 1.6.0, where I need to use some of the controls from AjaxControlToolkit. I downloaded the AjaxControlToolkit released on May 14th from http://ajaxcontroltoolkit.codeplex.com/releases/view/43475. I added the ToolkitScriptManager inside the MasterPage, and a CalendarExtender control in a User Control. I did not use any other javascript code on them and did not write any custome code as well. However, while running the website, I used to get the following JavaScript error each time I load a page that contains the User Control (where I added the CalendarExtender):

Error: 'Sys.Extended.UI' is null or not an object

I was searching for the solution on the web, but could not find any. I looked at the Web.config, and also tried changing the version of AjaxControlToolkit, but no use. After several hours of hard time and trying various solutions, I finally came across a post with perfect solution. What I needed was to add an extra attribute CombineScripts="false" to the ToolkitScriptManager tag. My code looked like:

    <asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server"
        LoadScriptsBeforeUI="true"
        CombineScripts="false"
        EnablePartialRendering="true" 
        EnableScriptGlobalization="true"
        EnableScriptLocalization="true">
    </asp:ToolkitScriptManager>

And BANG!!! It started working! I tried the page with IE, FireFox and Chrome, and it worked perfect in all of them.