How to fix magic quotes deprecated in php7 ?
Having checked the error log in the folder I see the reason for the 500 error was Magic Quotes runtime being required. The site was running on PHP7 and this feature was removed from PHP7 before release so that explains the error (http://php.net/manual/en/function.set-magic-quotes-runtime.php). Switching back to PHP 5.6 fixed this. But it cant be switched off for long.