Google reports problems with sitemap_index.xml

Hello,

Google reports problems with the sitemap generated with this plugin. Something is wrong with the lastmod-date.

<sitemap>
<loc>http://www.XXXXXXXX.de/post-sitemap.xml</loc&gt;
<lastmod>2013-06-24T21:59:34+00:00</lastmod>
</sitemap>
<sitemap>
<loc>http://www.XXXXXXXX.de/page-sitemap.xml</loc&gt;
<lastmod>2013-06-24T21:55:11+00:00</lastmod>
</sitemap>
<sitemap>
<loc>http://www.XXXXXXXX.de/post_tag-sitemap.xml</loc&gt;
<lastmod>2013-06-24 21:59:34</lastmod>
</sitemap>

These are the entries… and the last one is the problem. I don´t know why but Google reports there is a problem with the value 2013-06-24 21:59:34

Can you fix that please?

http://wordpress.org/extend/plugins/wordpress-seo/