I found out in webmaster tools that sitemap.xml gives an error on a certain line. This appeares to be following:
So in stead of <loc>the URL</loc> there is a <loc />
The rest of XML nodes are correct. It looks like this is a product that breaks things up because only a small number of products are shown in the sitemap and when I set sitemapincludeproducts to false in admin settings, webmaster tools produces no error.
Any idea what might cause this problem ?