I believe that you are using RadEditor Classic, because you should not experience this problem with RadEditor for ASP.NET AJAX. Could you please confirm that you use RadEditor.dll in your project's bin folder?
The latest version 7.3.6 of RadEditor Classic provides a new
configuration for the way information is passed to the dialogs, avoiding the
"session expired" message from the file browser dialogs. This functionality is
set by default and you shouldn't set any properties to turn it on.
You can find more information in following articles:
- Do RadControls use session state and/or cookies?
- Cookies and Session overflows with many Telerik controls. (RadEditor v4.x and 5.x)
If the problem still persists after the upgrade, remove the RadSpell.Net2.dll and test again.
You can also just for test upgrade your Classic version of RadEditor to RadEditor for ASP.NET AJAX
the Telerik team