Customizer not loading fully

Hello

After updating to v1.3.0 of the theme, the Customizer is not loading all the necessary sections, so I can’t change text in my main page.
Here’s a screenshot.

This applies to both the original theme and the child theme.
Also, this applies to normal operation or with all plugins completely disabled.
I don’t have any caching plugins.
I have cleared all cookies and history in two browsers (Chrome + Firefox).
Same result.
The customizer was loading properly with v1.2.9

I also tried using the code:
define('CONCATENATE_SCRIPTS', false);
and the code
remove_action('shutdown', 'wp_ob_end_flush_all', 1);
in functions.php but still no joy.

Any ideas?