Google Webmasters: Your Sitemap does not contain any URLs

Hello,
First of all thanks Yoast for this grate plugin!
I’m getting some errors on my Google Webmasters account regarding my sitemap.

Your Sitemap does not contain any URLs. Please validate and resubmit your Sitemap.

The error appears on the following files:
media_tag-sitemap.xml
post_tag-sitemap.xml
post-sitemap.xml

Is there anyone that could help me regarding this issue?
Our website is hotass.ro
Thanks!

https://wordpress.org/plugins/wordpress-seo/