To solive this issue you have to install the AjaxControlToolkit on your server
Server Error in ‘/’ Application.
Could not load file or assembly ‘AjaxControlToolkit’ or one of its dependencies. This assembly is built by a runtime newer than the currently loaded runtime and cannot be loaded.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.