Error "The requested script resource 'Common.Common.js' requires version 'AjaxControlToolkit, Version=4.1.60501.0, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e' of the ASP.NET AJAX Framework."

I am getting the below error on my application

The requested script resource 'Common.Common.js' requires version 'AjaxControlToolkit, Version=4.1.60501.0, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e' of the ASP.NET AJAX Framework. To use this resource, make sure that the application references version 'AjaxControlToolkit, Version=4.1.60501.0, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e'.

The application was working fine and all of a sudden we get this error. The application is running on .NET 4.0 using ajaxtoolkit 4.1.60501.0

Any help at the earliest is highly appreciated.