Hello,

It means that you have a bugged system plugin installed on your Joomla that add extra spaces at the beginning of the Joomla response, provided that no Joomla core files have been altered. Opening your standard html pages you will probably get that carriage return at the beginning too, although it's not a problem for HTML and browsers, as you may know it could be for XML and parsers.

Refer to this FAQ for the list of potential known bugged plugins: http://storejextensions.org/faq/xml-sitemap-results-not-well-formatted.html

Other than this, you need a quite meticulous debugging of your system plugins to identify the responsible of this extra carriage return.

Best regards

John