I'm struggling to find a solution to this problem. The following error has been showing up in my clients logs for some time.

PHP Warning: Cannot modify header information - headers already sent by (output started at /includes/templates/override/common/tpl_main_page.php:71) in /blog/wp-includes/feed-rss2-comments.php on line 8 and
PHP Warning: Cannot modify header information - headers already sent by (output started at /blog/wp-includes/nav-menu-template.php:235) in /blog/wp-includes/feed-rss2-comments.php on line 8.

I've looked for spaces and blank lines in both files and compared the files to the original Wordpress files. I cannot find any errors. I'm posting this in the support thread for WOZ because this headers already sent problem seems to be fairly common. My client is running WP 3.1, Zencart 1.3.9d, WOZ (copyright statements are from 2010), Simple SEP URL, and a number of other mods (I'm not entirely sure what or how many. Neither is my client.) I'm not terribly familiar with Wordpress and the common thread seems to be the rss2.php. I'm not sure what's triggering this or where to look to debug the problem. Can anybody help point me in the right direction?