The error seems to be related to failing to load an assembly from Telerik.
Could you check that the two assemblies "Telerik.Web.UI" and "Telerik.Web.UI.Skins" are present in your GAC. Also in the web.config file of your site, do you have an entry that looks something like "<add key="Telerik.Web.SkinsAssembly" value="DISABLED" />" under appSettings? If yes, you might have to change the value to "Telerik.Web.UI.Skins".
Here are some additional info from the Telerik forum that you may find helpful.