Invalid Last Mod date of root: how do you fix this?

I have this issue too. The error in sitemap on site, not only in Google.

I found in \wp-content\plugins\wordpress-seo\inc\class-sitemaps.php function get_last_modified. Search by it found that plugin get last_modifed_gmt from wp_cache, but it quicly died, else get from DB. posts, standart field.
Plugin works with date in format $date = date('c', strtotime($date)); Thus, it’s with letter ‘T’.
All looks right – code, table of DB…
But how fix error???

Probably, cause of the error was bad field in [database].[prefix]_posts.post_modified_gmt, this field had zeros instead datetime. Because, I does write posts directly to database with script, instead write it commonly with WP editor. Old version of the script didn’t write datatime to there.