here is what i’ve kind of figured out: the lighting is built by a separate process running on the same host machine - the editor needs to communicate with that process over a socket, and needs a particular port or ports open to do that. The ports are blocked by default unless they’re enabled by changing the mac firewall settings.
in my particular case, i’m hoping to convince the university’s IT staff to allow an exception for the ports we need.
for anyone else, check all the logs you can to see if it’s a matter of the editor timing out waiting for a connection to the Lightmass process - maybe your firewall was reset or maybe the rrequired ports have changed?
still in the dark (pun intentional)