Hi there @Rincat,
Thanks so much for all of the extra information and context.
You may want to try to enable WP debugging by adding:
define( 'WP_DEBUG', true );
to the wp-config.php file.
Here’s more information about WordPress debugging:
WordPress.org Forums – 4 Nov 18
Debugging in WordPress
Debugging PHP code is part of any project, but WordPress comes with specific debug systems designed to simplify the process as well as standardize code across the core, plugins and themes. This pag…
Once enabled, you may be able to see errors & notices that can help isolate the issue.
Let me know if that helps!
Sam