I indeed get that error Your Sitemap does not contain any URLs

But looking into the fetching and crawl stats from google there seems nothing to be wrong

The robots.txt is like this

User-agent: *
Disallow: /administrator/
Disallow: /backups/
Disallow: /cache/
Disallow: /cli/
Disallow: /components/
Disallow: /images/sampledata
Disallow: /includes/
Disallow: /installation/
Disallow: /language/
Disallow: /libraries/
Disallow: /logs/
Disallow: /media/
Disallow: /modules/
Disallow: /plugins/
Disallow: /templates/
Disallow: /tmp/

Any other idea what might be wrong?