I took a quick look at your site. I personally wouldn’t mess with the WordPress core files too much, but I don’t see any inherent issues in it.
I did take a quick look in your wp-config.php to check if debug mode was on. It was on, but I also noticed these rules:
// Disable display of errors and warnings
define( 'WP_DEBUG_DISPLAY', false );
@ini_set( 'display_errors', 0 );
These rules specifically disable any errors from being displayed. That’s why you get the completely blank 500 error page. If you want to see the actual error message, please remove or comment out these lines and refresh the page.