Following the provided information, I have tried to replicate the error locally:
However, I was unable to observe the same behavior on my end.
I have attached my sample project for your reference. Could you please give it a try and see how it works on your end? Do I need to perform any changes in the project?
Could you please ensure that the references to all Telerik controls in your project are the same version considering the suffix as well (.20 or .40). Better yet, you can remove all references and add them anew by using the DLLs from your latest installation. Delete the license.licx file. After that, you should rebuild your project, close Visual Studio and open it again to make sure that no references are kept in the memory by Visual Studio.
I am looking forward to your reply.
Dess | Tech Support Engineer, Sr.
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