Upon having uploaded 3.2.0-RC2 (the most recent version), it displays this error message and will prevent the installation of phpBB.

Fatal error: Uncaught exception 'ProxyManager\Exception\InvalidProxyDirectoryException' with message 'Provided directory "/tmp" does not exist' in /public/sites/www.mossingwa.com/rc2/vendor/ocramius/proxy-manager/src/ProxyManager/Exception/InvalidProxyDirectoryException.php:38"

Refreshing the page will make the error go away and let you continue with the installation. However, further in the installation process the error "Directory "tmp" can't be found" will come up again, this time definitely preventing install and stopping the process.