There is many posts about "XML or text declaration not at start of entity" error, and I also meet this error, I must steps by steps to open many php files to delete blank lines outside <?php and ?>, it is the heavy workload, so I develop a plugin to find blank lines and delete them from php files automatically.
Please try this plugin fix-rss-feed, it will fix your rss error automatically, and you can get more information from http://wordpress.org/extend/plugins/fix-rss-feed/, and you also can get help from the author website http://www.flyaga.info/en/wordpress/plugins/fix-rss-feed-error-wordpress-plugins.htm
author:flyaga li
email:flyaga@163.com