The error could happen because you and the developer used different versions of the RadControls assemblies. Can you ask them for theirs?
Here are some hints: is your site created by a Visual Studio WebSite Project, or a Web Application Project?
If the latter, you should either use a Binding Redirect
in the application web.config file, or you should recompile the project referencing the new assembly.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items