On line 102 of frontend/specialpages/actions/RequestAccount_body.php, a previous contributor added a typo:

$out->setPagetTitle( $this->msg( "requestaccount" )->escaped() );

This causes the site to throw an exception when a user visits Special:RequestAccount.

Correcting the spelling to $out->setPageTitle() instead of $out->setPagetTitle() stops the error from occurring.