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The Sitefinity isntaller and project manager come with their own version of the Telerik.Web.UI.dll. The version which was used to build the system. We try to upgrade the controls in the CMS frequently, but sometimes they are not the latest controls released.
The problem you are experiencing is because almost the whole UI of Sitefinity is built using RadControls for ASP.NET Ajax. This means that the Sitefinity assemblies reference the particular version of Telerik.Web.UI.dll which is in the bin of the Stiefinity website. When you have upgraded the controls from the VS dialog you have substituted the assembly with a newer one. To fix the problem you must either revert the assembly to in the bin folder or use a binding redirect
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