The problem occurs when you update your project to a newer version using the Telerik WinForms VSExtensions. It is a known issue and we are currently working on an improvement in the VS Extensions.
To resolve the issue you should install the Telerik.WinControls.UI.Design.dll
into the GAC. You can find detailed information about how to install an assembly into the GAC in the following MSDN article
I am sorry for the caused inconvenience. If you have any further questions, please write us again.
the Telerik team
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