Have you ever seen “AjaxToolKit is undefined” , propapply your website is hosted on Windows Server 2008 and IIS 7, and if you are using other script Manager likes teleriks you would see different message called “Telerik is undfined” or your javascript mauy not work at all,I found the simplest solutions is , whenever you are using
ToolKitScriptManager write CombineScripts="false"
If you are using RadScript Manager write EnableScriptCombine="false"