I am currently rebuilding whole engine.
But I think your problem lies with the heightmap itself. Please open one of heightmaps provided and look at the settings.
Mipmaps Setting = [NoMipmaps]
Compression Setting = VectorDisplacement (RGBA8)
sRGB= false
After that press reimport and your heightmap should work.
Press [Generate Bit Data from Texture]
after that [Horizontal/Vertical Optimization]
As soon as you see log that optimization if finished press [Build Blocks]
Good Luck.