This problem might occur if the RadEditor's skins are not loaded correctly. Could you please register the external RadEditor skins as it is shown in this KB article: Registering an external skin of RadEditor
You can also take advantage of the recently introduced CDN
support which will allow you to load the controls skins from the Telerik CDN. Telerik CDN is hosted on the Amazon CloudFront service.
You can find more information about our CDN support here
the Telerik team