We have recently been experiencing intermittent '500' errors on random websites which has been tracked to our current client side protection software - iThemes Security Pro. The errors have been randomly occurring when the .htaccess file is updated with black list entries, as happens frequently .
We have sourced an alternative software (Simple Securtry Firewall) which will continue to provide the same protection but doesn't write to .htaccess for black list management. We will be progressively switching out the software, following which you will see a new menu group - Simple Firewall. We recommend you don't adjust settings without checking with us as you could either end up locking yourself out or worse - allowing the 'baddies' in.
If you wish to read more about the features provided by Simple Security Firewall at https://wordpress.org/plugins/wp-simple-firewall/
We continue to also provide server side protection and malware scanning.
Ph: +644 974 8008