This error is fixed in version 3.x of RadSpell for ASP.NET. If you would like I can send you this build. Just let me know.
You can solve the problem in v1.x with the following workarounds:
1) set the UseSession property to true and control your ASP.NET session's lifetime from your application's web.config.
2) set QueryStringSessionData to true -- that will disable all cookies and session data and will pass the data to the dialog via the GET query string.
the Telerik team