I'm seeing the same malformed HTMl at the top of your source at http://www.soziocombustibili.it too, but not at http://domemassi.altervista.org/
Can you once more deactivate all plugins except the sitemap plugin at http://www.soziocombustibili.it switch to Twenty Fourteen, and leave it at Twenty Fourteen this time so we can take a look? Usually things like this are almost always caused by the theme.
