I guess the problem is in the assemblies, referenced by your project being a different version than the designer assembly you are attempting to use.
Can you please:
- make sure there are no Telerik.Web.UI assemblies in the Visual Studio probing paths (PrivateAssemblies/PublicAssemblies) and in the GAC
- Load your project in Visual Studio and rebuild it
- Check the version of the Telerik.Web.UI assembly in the BIN folder of your project
- Check if there is the Telerik.Web.Design assembly with the same version in your GAC
By the way, which Visual Studio version do you use?
Can you check the <assemblies> tag in your web.config as well? Is there the Telerik.Web.UI assembly mentioned there (in other words, do you have a GAC reference to the Telerik.Web.UI assembly)?
Looking forward to your reply,
the Telerik team
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