Error 500

I’m really not sure what’s causing the 500 error, that’s usually a server side problem. It would need a lot more of investigation to pinpoint the cause of the trouble.
First of all, how many posts with featured images do you have?

Try enabling error displaying or logging so that we can know more about the issue:
Either switch that ON via your hosting control panel or set it via .htaccess or even better through WordPress itself; put the followng in your wp-config.php file:

define('WP_DEBUG', true);

or via PHP directly:

ini_set('display_errors','On');
 error_reporting(E_ALL);