I have the exact same issue, and it started happening when I updated both the Plugin and WordPress to the latest version.
Manual update works fine, but auto update does nothing, and I get the same message as the above poster.
I have tried deleting my existing sitemap, I tried the uninstall and fresh install but to no avail.
Hopefully there will be a fix in the very near future.
Duke
I had the exact same problem / error, although mine was with a fresh install.
All I did was clear my cached files and history, refreshed and the error has gone..
Hope this helps others..
The Duke
