We experiencing the 500 Error with WordPress sites mainly access themes/plugins in the wp-admin dashboard.
This is mainly due to the issue with Memory Limit of WordPress.
Adding the following line in the wp-config.php will resolve the issue.
define('WP_MEMORY_LIMIT', '64M');
Just add the line under the PHP tag.
<?php
define('WP_MEMORY_LIMIT', '64M');
/**
The base configuration for WordPress
*
The wp-config.php creation script uses this file during the
installation. You don't have to use the web site, you can
copy this file to "wp-config.php" and fill in the values.
*
This file contains the following configurations: