I’ve been going through the process of getting a project setup on my machine that has recently been upgraded from Sitecore 6.5 to Sitecore 6.6. Mostly things have been uneventful, but then I tried to load the Content Editor.
I encountered the following error caused by the Telerik controls:
Telerik.Web.UI.RadToolTipManager with ID=’ToolTipManager’ was unable to find an embedded skin with the name ‘Telerik’. 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.
A new DLL (Telerik.Web.UI.Skins.dll) has been introduced to provide the Skins for Telerik controls, and this is a must-have for your development on Sitecore 6.6. Just make sure you have that DLL where you previously had Telerik.Web.UI and you should be golden!
Telerik’s website has more information on Skins troubleshooting