When trying to convert a static mesh to Nanite, if that static mesh has more than 64 material slots, the Engine crashes. There are MANY threads in the forums about this issue, and most indicate the root cause of the crash is that there are too many material slots. While that may be true, the Engine should be smart enough not to try to convert such a mesh in the first place.
I believe the crash is originating from the inline-allocated array in Nanite::FCluster::BuildMaterialRanges() – there should be a bounds check there.
As for why anyone would need more than 64 material slots, there are a few issues with the new FBX interchange importer in 5.5 that can result in meshes containing a LOT more material slots than desired.
First, if an FBX contains multiple mesh objects, the new FBX interchange importer seems to import some of those objects with the wrong names. The workaround for that issue is to use the “Combine static meshes” option. When that’s used, though, each static mesh ends up with a superset of all the material slots of all the objects in the same FBX, easily exceeding the 64-slot limit of Nanite.
Second, turning off “Keep sections separate” during FBX import can reduce the number of material slots, but then I find that the importer tends to link multiple faces of meshes to the same material when it should not.