I'm having problems using XML google sitemaps with the plugin:
However, google bothers about an xml generated by a firsturl.com containing secondurl.com/links.
SO, I suggest you to check if this plugin is active and, in case it is, to use the primary domain for the website.
Actually, in a website with 14 domains mirrored, your plugin is using a domain in the middle that nothing have to do with the "default" one.
Google error is:
URL non consentito
Non è consentito utilizzare questo URL per una Sitemap disponibile in questo percorso.
That is URL not usable
It is not allowed to use this URL for a sitemap on that path.