robot.txt and xml sitemap errors

Hi,
after latest update i got a message from google webmaster tools that it cant download my robots.txt file.
in addition i cant see the sitemap_index.xml content (get a blank).
yes I am using total cache, and i have read all posts by yoast on the subject but non of them fixed it.
any ideas? this happened only after latest update.

thanks

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