This error means that the Image Editor dialog was unable to submit its data to the server correctly. The image data is submitted using a simple hidden <input> HTML element.
What you need to do is check your application for any modules that might be modifying the posted values of HTML forms. If you have upgraded your application with a more recent version of the Telerik.Web.UI assembly and are using the ExternalDialogsPath property of the RadEditor, check if you have also updated the EditorDialogs folder to the latest version. Finally, you can use a HTTP debugging proxy (Fiddler for IE, Firebug's Net panel for Firefox) and examine the posted forms data when you click the Save button in the Image Editor dialog. Look for the "imageData" value.
the Telerik team