Remarks
If an XML declaration (<? XML …?>) appears in your feed, the first thing in the diet, especially a room.
Unfortunately, with WordPress, it seems too easy for a plugin, theme or your configuration file to a blank line. Compounding this problem, some – but not all – feed readers compensate for this common error, the error may go unnoticed for a while.
Solution
* PHP also uses a similar format: <? php … ?>. Note that the final php?> should be removed from all PHP code files modules, includes, etc. The final separation stabbing is optional in PHP (but not XML), and removing it helps prevent unwanted white space at the end the files can cause problems with your feeds.
* Check your wp-rss2.php and WP atom.php for blank lines outside of <? and?> parentheses sections.
* Check your wp-config.php file for blank lines outside of <? and?> parentheses sections.
* Check your themes functions.php file for blank lines outside of <? and?> parentheses sections.
* One by one, disable plugins and rehabilitation until the cause of the problem. ЁЯЩВ