Viktor Nagornyy on "[Plugin: iThemes Security (formerly Better WP Security)] 500 Internal Server Error [Urgent help]"

Please check your htaccess file, make sure you have nothing there but the default WP code:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

Issues with htaccess usually cause 500 server error. Hope that helps :)