Hello,
Thanks for your answers guys, I have been able to understand what is happening with the crash, it is actually not related to a LightmassImportanceVolume, but more like unused material in my mesh.
I am working with Cubegrid to design my level, which creates a static mesh, but sometime there is a material slot that is set to “none”, and this makes the GPU Lightmass crash. I couldn’t find a way to remove an unused slot with the “Edit Material” (of the modeling tool) without disorganizing the order of the material slot, which is a pain if I have multiple slots.
I also made a video to explain exactly what happens : https://youtu.be/sGUEMLDFWew