The issue is most probably related to the fact that you are using an old version of the Telerik.Sitefinity.Silverlight.xap
Silverlight application, found in the /ClientBin
directory of your site.
To work around that, you may create a new project with the latest Project Manager and copy the files from the /ClientBin
directory of the newly created project to your existing project's /ClientBin
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items