You could experience this problem after an application restart. The toolbar image sprite used by the different tools is loaded from the embedded resources in the Telerik.Web.UI.dll which and has a dynamic url. When the application is restarted, the web browser is trying to load the old invalid path to the embedded resource, but not the newly generated one.
You can easily fix this browser caching problem, by registering the skin of RadEditor from a CDN server
or the external skins
of the control which have fixed paths.
the Telerik team