Please check whether you use Telerik.Web.UI
assembly built against a higher version of the .NET Framework than the Target Framework
of the current project. This most often happens when a project, targeting .NET Framework 3.5 has been created in Visual Studio 2010 and it has been populated with the Telerik.Web.UI assembly in the Bin40 folder.
- What's the TargetFramework setting of your project?
- What's the version of the Telerik.Web.UI assembly?
- What is the .NET version used by the application poll that runs the website?
- Check the web.config file and whether you have set specific version for Telerik.Web.UI
- Check whether you have some reference to Telerik.Web.UI from the GAC.