Once again, one of my clients was getting the XML space error with the RSS Feed and Google Feedburner. Seems like this happens every other time WordPress upgrades.
Here is an example of the error message
The URL does not appear to reference a valid XML file. We encountered the following problem: Error on line 2: The processing instruction target matching “[xX][mM][lL]” is not allowed.
To fix it we discovered a few new ideas at http://validator.w3.org/feed/docs/error/WPBlankLine.html
First, make sure the there is no ?> at the end of any <?PHP phrases (even if it is at the end of the file).
Next delete all extra lines at the end of the file.
Most significantly, error stopped after fixing the /feed/ file