Who do you host with?
That is an odd page to throw an error. All that page does is unsets the session variable used to keep track of the theme you are currently editing and redirects you to the admin design page.... here is the actual code for that page (it is only 3 lines of code):
if (!session_id()) session_start();
I can't imagine how that would cause the error it does on your server. I'm thinking it has to be an issue with the php server itself and how it handles sessions, but it is hard to say. I'll open a support ticket and we can give you a call and try to help you debug it directly.