I'm getting the same error in Google Webmaster Tools.
Missing XML tag. This required tag is missing. Please add it and resubmit.
Thinking there was a post for that year month range, but it's been deleted. Must still be in the database somewhere. Only thing i can assume is causing this problem. Otherwise the plugin is working fine on another blog.
Looks like Google will not index this sitemap with the error, waiting for me to correct the error and re-submit. Guess i will have to switch back to the last version of this plugin until i can figure out whats wrong.