I noticed the same error. Looking at the sitemap file, I discovered that the format of the date has changed:
correct entries show:
<lastmod>2013-06-25T07:37:32+00:00</lastmod>
incorrect entries look like this:
<lastmod>2013-06-20 19:25:06</lastmod>
I'd like to have the "T" instead of a blank. :-)
What has changed lately on my site rro:text
- updated the plugin
- updated to wp 3.5.2
Thank you in advance.
Ruth