I had the same issue with WP 3.9.2 and the italian locale (WP_LANG set to it_IT),
and the 1.0 plugin. My sitemap_index.xml looked like:
[...]
<sitemap>
<loc>https://[mywebsite.com]/pods_faq-sitemap.xml</loc>
<lastmod>2014-08-14 13:30:56</lastmod>
</sitemap>
[...]
The problem is the format of the date (not UTC).
I inserted this line in the classes/pods-seo-wpseo.php file, at line 228:
$lastmod = gmdate("Y-m-d\TH:i:s+00:00", strtotime($lastmod));
Then the error/warning on webmaster tools disappeared.
Michele