The system: WP 3.8.1 MU, with Domain Mapping and your plugin 4.0.b11 version.
The problem: One site (with one domain, not subdomain) sitemap create the plugin. I look that in my browser. I'm add that sitemap in Google Webmaster Tools and test it. The google wmt write that error: Empty sitemap.
Anyone site working. This is the only bad sitemap: http://www.shisharoma.hu/sitemap.xml