I set the settings for the lightmap as recommended in the lightmaps section of the README, but Unity is still giving the texture HDR compression instead of the required DXT5 compression. I get a similar result with the shadowmap - Unity gives it ARBG 16 bit compression instead of the required DXT1. Having these textures in the wrong format is of course preventing the level export.
I am using Unity Personal 2017.4.17f1 LTS.
Here is a screenshot showing that the texture settings match the README:
