Hello Sam,

It is possible that there are cases when there are issues related to the upgrading/installation process. By  the information that you provided it seems that the project/application that you are attempting to run can not find reference to the Telerik.Web.UI.Skins.dll assembly file where all of the predefined skins (except for the Default one) are located. There are two reasons for the exception to be thrown - there are either no references set to the upgraded assembly files or the references are still the old ones pointing to the GAC. To resolve the exception you can try using the following approach - place all of the assembly files (Telerik.Web.UI.dll, Telerik.Web.UI.skins.dll and Telerik.web.Design.dll) in the local bin folder and update the references as demonstrated in this video.

Or you can use the following approach as well:
1. Delete the existing ones
2. Add the the Telerik.Web.UI.dll, Telerik.Web.UI.skins.dll and Telerik.web.Design.dll in the Bin directory
3. Add references to the above mentioned dll files.
4. Rebuild the solution

Give it a try and let me know how it goes.

Kind regards,
Kate
the Telerik team