Yoast Sitemaps Issue

Is anyone else having issues with Yoast site maps lately? I’ve googled around and no one else seems to be reporting the problems I am getting lately (some posts from years ago look similar though).

In Google Webmaster, I am getting:

# Type Issue Description Issues count Example Line Detected
1

Errors
Invalid date
An invalid date was found. Please fix the date or formatting before resubmitting.
11
Parent tag: sitemap
Tag: lastmod
61
Apr 8, 2014
Parent tag: sitemap
Tag: lastmod
65
Apr 8, 2014
Parent tag: sitemap
Tag: lastmod
69
Apr 8, 2014

I can’t resubmit the site maps because of this error.

As you can see here: http://thpstockphotos.com/sitemap_index.xml – All the “tags” site maps other than the first and the last never seem to have the Last Modified date attached to them, which appears to be the problem.

I have turned site maps on and off, updated to latest version, all to no avail.

Anyone else?