IPFire DNS is not resolving DHCP names

Something has changed recently (139?) and I can no longer resolve names of DHCP clients. What do I need to look for / reconfigure to get this working again?

Apparently what changed was “Enable DNS Update (RFC2136)”. It got checked and I’m not sure if it was an operator error. With this checked the daemon unbound-dhcp-leases-bridge doesn’t get started.