The reason for the error you are getting could be due to incorrect loading of the UserControls. If you are switching UserControls before the ViewState is loaded, ASP.NET will try to load the ViewState from the previously loaded user control into the new one. Thus, as the controls collection of the two does not match, an exception will thrown.
For example if you are changing the userControl on a button click event you should load the new UserControls inside the button's click handler and use Page's Init event to load the same UserControl which has been loaded in the previous page request.
I would also suggest you to review the following online resources which elaborates on Loading UserControls with Ajax:
All the best,
the Telerik team
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