Have you been able to solve the issue, we are facing same problem on upgrade v6.x to 8.2.x but when we try to edit any news item we are getting;
Telerik.Web.UI.RadWindowManager with ID='windowManager' was unable to find an embedded skin with the name 'Sitefinity'. Please, make sure that the skin name is spelled correctly and that you have added a reference to the Telerik.Web.UI.Skins.dll assembly in your project. If you want to use a custom skin, set EnableEmbeddedSkins=false.
error and when we put
<add key="Telerik.EnableEmbeddedSkins" value="false" />
Web.config, problem disappears but we are unable to buttons on the editor side
And the solution in here is not working also;
any help would be appreciated.