Las tengo añadidas al final del resto de reglas:

RewriteRule ^sitemap.xml$ auto_sitemap/index [L]
RewriteRule ^sitemap.xsl$ mod/auto_sitemap/views/default/auto_sitemap/sitemap.xsl [L]
RewriteRule ^sitemapindex.xsl$ mod/auto_sitemap/views/default/auto_sitemap/sitemapindex.xsl [L]

Hol Javier. Pues parece que en tu .htaccess que esta causando esa redirección antes de llegar a las lineas del plugin. Quizás prueba a ir subiendo las lineas en el .htaccess . Prueba y error ... :/ Enlugar de ponerlas de ultimas vete probando poco a poco a ver si das con la regla que hace la redireccion

Solucionado!!, el problema era causado por el plugin Login_required, que estaba limitando el acceso a la url /sitemap.xml, además del resto de urls: 

auto_sitemap/group/ 

auto_sitemap/blog/

...

También he tenido que hacer públicas las url del contenido para que Google pudiera indexarlo.

@the_yke muchas gracias por guiarme al camino de la solución.

Perfecto! Gracias por compartir la solución

Has anyone this plugin installed in NGINX?, can you share the rewrite rules??

Thanks in advance!

i am just using this for nginx:

    location = /sitemap.xml
    {
        return 301 https://www.yourdomain.tld/xml_sitemap/;
    }

@ura soul I get a 404 error with this rule when I go to http://mysite.com/sitemap.xml

assuming you changed the yourdomain.tld part fo the configuration that i shared to match your site's domain name and extension, then i'm not sure what the problem is.
is there any useful error logged to the nginx logs?

No errors logged in logs, only a page with: error:404:content. This is the url of my test server:

red.triploader.com/sitemap.xml 

After some tests, It is working with:

 location = /sitemap.xml {

                return 301 http://$server_name/auto_sitemap/index;
 }

ah ok, maybe i am using a different version of this plugin - there is more than one in the community.

Cant seem to get this working - any know issues with 1.9?

@knussear I´ve just get it working in my Elgg 1.9.8 installation. The only issue is this plugin conflicts with Developer_tools plugin, it removes this required line:

<?xml version="1.0" encoding="UTF-8"?>

will this plugin be available for 1.9 ?? or do anyone has a fix for this.. I deactivated the developer plugin but it's not seems to be working..  

Which is the problem? Have you added the needed rewrite rules to your server? 

I deactivated developer tools in 1.9.8 and added the Rewrite Rules to the .htaccess. It worked in 1.8, but does not work anymore. The site can not be found, when I try to call www.mysite.com/sitemap.xml

How did you get it work, Javier?

@Hanzolo with the right rewrite rules and Developers plugin disabled for me is working (now in Elgg 1.10). Maybe you have walled garden enabled o something limiting logged out user access?

i have just upgraded this to 1.9 - 1.12 compatibility. shall i upload it to my own listing in the community?

yes please uara soul, I will not mantain this plugin any more. Fell free to extend and share it