As reported in message "Google Sitemap - Invalid XML attribute" the generated sitemap files give an error when imported by Google. The message is Line 3. Status:
Invalid XML attribute. The XML attribute of this tag was not recognized. Please fix it and resubmit.
After checking with http://www.sitemaps.org/ the header of the generated sitemap file is changed to reflect the header as specified here. Attached are the new index.php file (which replaces the index.php file in modules/sitemap) and the patch file with the changes in the index.php file.