Originally reported by Ulfr on IRC.
/ConfirmAccount/business/ConfirmAccountPreAuthenticationProvider.php: Call to undefined method MediaWiki\MediaWikiServices::getDBLoadBalancerFactory()
There's indeed a call to MediaWikiServices::getInstance()->getDBLoadBalancerFactory(); on line 117:
But getDBLoadBalancerFactory is not defined on MediaWikiServices nor its parent class:
https://phabricator.wikimedia.org/diffusion/MW/browse/REL1_27/includes/MediaWikiServices.php