Hello Bastian John,
A single application cannot reference different versions of the same assembly, and this is a general .NET limitation that applies to our controls as well.
Thus, I advise that you ensure your app references a single version of the Telerik assemblies and that the Telerik.Web.UI.dll and Telerik.Web.UI.Skins.dll versions match.
You can use a bindingRedirect element to forward requests for old versions to the current if you cannot remove all pointers to them (e.g., Register directives in user controls). Here is a basic example:
I also advise that you review the following article on using the GAC because it also shows how GAC references look like which will help you ensure they are correct: http://docs.telerik.com/devtools/aspnet-ajax/deployment/using-the-global-assembly-cache.
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 Feedback Portal
and vote to affect the priority of the items