I believe that this is a kind of dependency issue, you can try the following:
1. Make sure that all referenced UI for WPF assemblies or of the same version (XAML or NoXaml, release version, .NET version of the dlls
2. Make sure that the .NET version of the Telerik dlls
is the same as the Target Framework of the VS project.
3. You can remove all assemblies and add them again from the installation folder of the UI for WPF suite.
Do you need help with upgrading your AJAX, WPF or WinForms project? Check the Telerik API Analyzer
and share your thoughts.