I have the exact same problem, and I'm not using multisite.
Google Webmaster Tools Tells me about a 404 error, but when I check my sitemap, there is no problem.
The thing is : when there is "www." before the adress of my site, i can see the sitemap. But if it's "mysite.com/sitemap_index.xml", it works fine.
"www.mysite.comsitemap_index.xml" doesn't work (I get a blank page).
It was working good for month until 3 days ago. How to fix that problem without switching to another plugin ?