Yesterday I checked my Google webmaster tools profile and I find out that all the urls of my kentico-based portal had an error of kind "Url not allowed - this url is not allowed for a sitemap at this location".

After googling I found that if the sitemap file/handler is in a subdirectory (in our case /SpecialPages/GoogleSitemap.aspx) Google allows only <url location> that start from that subdir and below.
See this post for more info:
http://gsitecrawler.com/en/faq/info/google-sitemaps-err/

After moving the page in the root of the site, the crawling was ok.

Only to let know to other people with this problem