After hours of debugging this, I got it work.
I have no idea why, but it’s the static mesh (SM_FogSheet_Plane.uasset). If you move it inside of UE4, duplicated, rename it, or whatever, it scrambles something to do with the FBX source path. So go ahead and migrate the whole group of files to where ever you want in your own project. Then manually copy this one file from the original Blueprint project to your new directory location (overwriting the migrated one). Re-link it to the BP if necessary.