When the RadEditor is set to ContentAreaMode="Div" mode, opening the Find And Replace dialog and triggering the ReplaceAll functionality is throwing a javaSCript error.

A possible workaround is using the provided FindAndReplace.ascx user control in the attached sample project as external dialog. For more information about this approach you can follow this online demo:
http://demos.telerik.com/aspnet-ajax/editor/examples/externaldialogspath/defaultcs.aspx