Hello Heike,
Thanks for writing in! This error usually occur if you have modified the contents of the functions.php file. Please login to your ftp and browse to the theme’s folder wp-content/themes/x-child/ and edit the functions.php file. The content must have this code:
<?php
// =============================================================================
// FUNCTIONS.PHP
// -----------------------------------------------------------------------------
// Overwrite or add your own custom functions to X in this file.
// =============================================================================
// =============================================================================
// TABLE OF CONTENTS
// -----------------------------------------------------------------------------
// 01. Enqueue Parent Stylesheet
// 02. Additional Functions
// =============================================================================
// Enqueue Parent Stylesheet
// =============================================================================
add_filter( 'x_enqueue_parent_stylesheet', '__return_true' );
// Additional Functions
// =============================================================================
If still nothing is working, please provide us the actual contents of your child theme’s functions.php file in your next reply.
Regards.