As I ran into this issue on macOS and was also unblocked by disabling my VPN, I thought it would be helpful to shine a bit more context into this thread.
On macOS (and I believe Linux as well), Unreal Engine uses the class FSwarmInterfaceLocalImpl to launch a subprocess named UnrealLightmass in order to precompute lightmaps for static and stationary lights. Both the parent UnrealEditor process and the UnrealLightmass subprocess use this same Swarm interface local implementation (not to be confused with the Swarm Agent for Windows) to communicate with one another using UDP multicast messages over group address 230.0.0.1:6666.
This means that networking software like firewalls and VPNs may contribute to a breakdown in communication between these two processes resulting in light build failures.