Lightmass build stuck at 99%

I have a build with many objects and quite a few (static) lights.

Building the lighting usually took a few hours, without any problems - but after my latest changes the lightmass seems to be stuck at 99%. I tested at preview quality and that completed just fine.

If I look that the output log, it seems that all objects have been built (there are some regular warnings for overlapping lightmaps which I can safely ignore, all of these have been reported). This is in line with the 99% that I’m seeing.

So my question is: what processes are done by lightmass after all lightmaps have been calculated? There must be something that can cause this?

To my knowledge the last 1% of lightmass building is generating the actual lightmaps from the data calculated in the last 99% it being stuck on 99% usually means you have quite large lightmap resolutions or lots of uv islands which take longer to compute.

It is most likely still running, give it more time.