run time error while executing asp.net website

I had faced the same problem and I had it solved too.
I had done the same thing as mentioned in Solution 1 by E.F. Nijboer.
Just remove and add the same dll reference of the AjaxControlToolkit.dll and you'll get it working!
There is no Framework problem and also the version of AjaxControlToolkit that you are using is Correct..... It's just some another unwanted error...

First register ajax control toolkit in your page

<%@ Register assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" tagprefix="ajx" %>

Then add this code anywhere inside aspx page

<ajx:toolkitscriptmanager id="ToolkitScriptManager1"  runat="server">

i think the error cause by your machine don't have .NET framework 4.0 or maybe you dowload a wrong version of AjaxControlToolkit :)