The Kendo UI Grid for ASP.NET MVC data source uses a POST request by default when it is Ajax() bound, so it should not be necessary to add the AllowGet in the controller.
If you need further assistance, it is better to provide a code snippet of the entire grid and data source as well as the controllers.
As for the client side error, are you referring to the dataSource change event or the grid change event?
Let us know in case you need help with that.
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