Error:

PHP Fatal error:  Uncaught Symfony\Component\DependencyInjection\Exception\LogicException: %scheb_two_factor.security_voter.class% should implement the Symfony\Component\Security\Core\Authorization\Voter\VoterInterface interface when used as voter. in vendor/symfony/symfony/src/Symfony/Bundle/SecurityBundle/DependencyInjection/Compiler/AddSecurityVotersPass.php:54

This service is defined in https://github.com/scheb/two-factor-bundle/blob/master/Resources/config/security.xml#L52. Looks like class is not resolved in https://github.com/symfony/security-bundle/blob/master/DependencyInjection/Compiler/AddSecurityVotersPass.php#L45.
It returns parameter instead.
Error is going away when I hardcode FQCN instead of %scheb_two_factor.security_voter.class% parameter.

I've read https://github.com/symfony/symfony/blob/master/UPGRADE-3.4.md#securitybundle and this class does implement VoterInterface, the problem lies in fact that AddSecurityVotersPass is operating with "%scheb_two_factor.security_voter.class%" string instead with FQCN.

/cc @nicolas-grekas