It seems that the problem comes from having a different version of Telerik.Web.UI in the GAC but I will need some more information about the problem in order to be able to help. Would you please provide the following information:
When you open the demos, do you receive the error only for this online demo or for all the demos, too?
Does the problem occur when you want to use the demo in a separate project or it occurs when opening the solution of our demos?
Please provide a detailed explanation of your actions - what exactly you do, can you initially run the project, is the error thrown after you perform some actions on the page or at compilation time?
Kind regards,
Svetlina
the Telerik team
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