The 403 error means that the custom dictionary file is not accessible. The spell control needs to update this file, but the ASPNET process does not have write rights on the dictionary folder. The folder is located at
\Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\18.104.22.168__1f131a624888eeed\RadControls\Spell\TDF
Try giving the NETWORK SERVICE account write permissions for this folder. If that does not work, try giving write permissions to the currently logged in MOSS user (if using windows authentication).
the Telerik team