Have you tried running a "vanilla" call to sitemap friend?
[[sitemapFriend? &type=`xml`]]
If you run that and it correctly generates the output, then you can look to your chunk "sitemap_xml_item_tpl" for the error.
If that doesn't pan out you might want to verify that the setting "Container Suffix" is set to "/" under Manager > System > System Settings > Friendly URLS
Hope this helps