All lights were taken out including volumes etc., world settings were returned to default and still the persistent error “lighting build failed” ?
Do you get the same error when you try to build one of the default template maps?
Could you post the log of “swarm” so that we can see why the light build fails
which engine version do you use?
what do you have in your scene?
It’s very strange that the light does build but just gives off the msg every time.
Using 4.9.2, and no idea how to use swarm.
Still haven’t managed to fix it but … Started a new project and copied the directories from the current. Without the Maps copied it works fine, but with the maps gives off the error, can’t even create a new map from scratch to see where it’s going wrong. Must contain some preference values.
Don’t know if that helps.
Thanks for the reply so far.
What happens when you just copy everything in your map into the new one + try it again? 
So just a blank one + then paste everything into that
Does the Map load any World settings with it ?