Blank page on upgrade from 2.1.4 to 2.2.x

My web application is working fine on PHP 5.6.6 running CI 2.1.4. upgrading to CI 3.0.0 breaks a lot of the code so I'm wanting to upgrade to the most stable legacy version (2.2.2) however when I follow the upgrade instructions, my web app just loads a blank page with no source and no error log.

Working web app:
PHP 5.6.6
MySQL 5.5.x
CI 2.1.4
MySQLi driver

Not working
PHP 5.6.6
MySQL 5.5.x
CI 2.2.0 or 2.2.2
MySQLi driver

I've backed up the old system directory and replaced it with the new as well as the same with the index.php

Ideas?