Please check whether all telerik binaries in your project have the same version.
Have you upgraded your telerik version recently? I can suggest you to try several things:
1. Go to the location where the telerik controls are installed on your machine and remove the Design folder. Clear your GAC and re-open your solution and check whether the same error still occurs.
2. Try to uninstall and install the telerik controls.
Please let me know if one of these helps you.
All the best,
the Telerik team
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