Looking at the code on the page I'm not sure why you would be getting this error. I did not see the <? after line 67 on the page you posted, there might be something going on there.
Have you tried using any of the simpler session destroying code, like just session_destroy(), or unset($_SESSION)?
I think it might be a good idea to use the simpler code to destroy the session rather that the script you currently have in place. I think it is a bit overkill. Give this a try and see if it clears up the errors you are getting.