I have the same issue.
And after I delete the old sitemap like it tells me, the new one it generates is all weird.
The old one correctly listed the pages on my site:
domain.com/post1
domain.com/post2
domain.com/post3
etc.
THe new one just links to more .xml files:
domain.com/sitemap-pt-post-2014-01.xml
domain.com/sitemap-pt-post-2014-02.xml
etc.
And then when I click on those I get taken to another xml file tht links to the actual post.
That seems like it’s not genetating it correctly.