Hello,
Such may happen when you implemented the customization in the wrong way. First of all, make the full backup of your current theme and try to move the code into the proper places: all the custom CSS > Theme Settings > Custom CSS of the child theme; all the custom JS to the Theme Settings > Custom JS; all the custom PHP > functions.php of the child theme.
Next, check all your code for errors. Make sure the code is clean and valid.
Then create the header in the Header builder if you use the Header in the Theme Settings > Header.
Deactivate all the plugins not realated to the theme and try to update.
Then activate your plugins one by one checking how the child theme works.
Best Regards