gasoved on "Sitemap xml feed is shown but 404 header added by WordPress if site has no posts"

Here comes a correction: At first I thought it worked, because started to show 200 OK headers randomly, but not related the hotfix above, not sure if it was browser cache or something else.

I found the source of 404's though, it's Batcache plugin. I added a line to disable it for site map URL (just for parent site map (sitemap_index.xml) was enough) and it worked. Like this:

if( strpos( $_SERVER['REQUEST_URI'], 'sitemap_index.xml' ) )
	return;