/wp-admin/admin-ajax.php Error 500 while using divi premade layout in existing pages

Increase the memory_limit in the wp-config.php by adding:

define('WP_MEMORY_LIMIT', '256M');

Or,

go to your wp-config.php file in your root directory,

Change this line:

 define('WP_DEBUG', false);

to;

 define('WP_DEBUG', true);
 define('WP_DEBUG_LOG', true);
 define('WP_DEBUG_DISPLAY', false);

this will log any errors you’re having with additional information in wp-content/debug.log file.

From your comment, it seems a process or theme/plugin is using too much memory. Deactivate inactive plugins. (Increase the memory to 512mb to enable you troubleshoot)