Hi,
Reading the PHP error log file is essential - any 500 errors should have an entry logged there, which will identify the issue.
However, if you're not sure how your 500 errors are ending up in a themed page (only front-end pages in WordPress get themed - the back-end is not themed), then that should be investigated too, as it seems that there's something else going on in your setup that isn't what you're expecting.
You could also try increasing the memory limit, in case it's that (probably not, but worth a try): put this in wp-config.php:
define('WP_MEMORY_LIMIT', '512M');
Best wishes,
David