Also, the date format for the <lastmod> parameter is wrong (According to Google). They want the full ATOM format not just Year-Month-Date. I modified csg_functions.php this way:
197:
<lastmod>'.get_the_modified_date( 'Y-m-d\TH:i:sP', $id ).'</lastmod>';
Also, the Additional Pages entries don’t get a modified_date like pages or posts do. The output to the sitemap is empty for these: <lastmod></lastmod>.
Just to make Google stop throwing errors I have set today’s date (whatever day the sitemap is updated) as the date for each extra url:
227-230:
// Additional Pages don't have a modified date. Add today,
// as Google needs some date, not an empty node
$md = get_the_modified_date( 'Y-m-d' );
if($md == '') {
$md = date( 'Y-m-d\TH:i:sP', strtotime('now'));
}
$csg_sitemap_content .= '<lastmod>'.$md.'</lastmod>';
if( changeFreq() != 'hide' ) $csg_sitemap_content .= '<changefreq>'.changeFreq().'</changefreq>';
$csg_sitemap_content .= '</url>';