Hmm, judging by your reply that didn't solve the problem then.
Just wondering, in the bootstrap.inc (where I suppose your session setup is in), does it have the
<html>
<head>
etc? Cause I don't see that in the code.
But as long as there are no blank lines in the bootstrap, then I'm as puzzled as you are. Unfortunately I'm not an expert, so I'll leave this to the good guys if you don't mind =)