The error you are getting could be a result of just some sort of output occurring, or a problem with the session. To rule out a problem with the session please use these two test pages that I created that will set a value and print it to the screen.
Please load these files and test them and let me know what the result is. If you get the same thing on both pages then the sessions should be working ok. If this is the case please post back with a copy of the survey page in question.