The sitemaps at the lower level, like ".../page-sitemap.xml" have no faults. The top level one ".../sitemap_index.xml" has a problem with an invalid data.
In Dutch (i'm sorry guys) the index error is:
Bovenliggende tag: sitemap
Tag: lastmod
Waarde: 2013-06-23 17:43:46
I have a new site, so indexing should hopefully happen for me. If not it's probably because of this index error.
Hope it is solved soon. I love the plugin.