Problem with "headers already sent" errors

I've been developing on a Mac (DW CS5, eCart 5) and testing on a Linux web server.
The site has been working fine on both platforms.

In the past few weeks I've made a few changes. The site is still working fine on my Mac development platform (I use Apple's apache) but when I uploaded the new chagnes to the Linux test server I have been getting errors on some pages about "headers already sent".

I looked up some info on the web and found a lot of talk about not having spaces or even line returns between PHP tags, or else making sure start_session() happens at the top of the page. I tried both things, and they seemed to help fix the problem (the error stopped showing up).

Than, later, for various reasons I ended up taking those changes back out (I put line breaks between php tags), and strangely enough the pages continued to work without showing errors.

Now, however, I have one last page that is showing the "headers already sent" error and I cannot fix it. I'm really not able to remove all the spaces and returns for the whole page all the way down to where the problem is (line 291).

The exact error I'm getting on this page (checkout_success.php) is:

Warning: session_regenerate_id() [function.session-regenerate-id]: Cannot regenerate session id - headers already sent in /var/www/clients/client5/web9/web/checkout_success.php on line 291

The file is attached.

Can you please let me know what I can do to fix this error?