What does the error say?
I suppose that RadScriptManager combines the MicrosoftAjaxWebForms.js and MicrosoftAjax.js files and serve the compressed output. What you can try is to disable compression:
<telerik:RadScriptManager ID="RadScriptManager1" runat="server" OutputCompression="Disabled">
Let me know if this helps.
Also, if there is a public URL where we can observe the problem, that would be helpful.
the Telerik team