The child theme is corrupted. I can't tell you which part is corrupted because we need FTP access to check. For the mean time, try to download a fresh copy of X child theme from your dashboard. Extract it and compare the content of that to the content of your child theme on your site. Inside child theme folder, functions.php and style.css is important. The following part of the child theme style.css file shows how it reference to main X theme:
Theme Name: X – Child Theme
Theme URI: http://theme.co/x/
Author URI: http://theme.co/
Description: Make all of your modifications to X in this child theme.
Hope this helps.