I have this with various landscapes in my world composition level.
When ever they are visible, the game become extremely slow due to a lot of error messages printed in the log, in the form of:
LogTexture: Error: Texture2D /Game/MyGame/Maps/World/Level09/Level_09_Terrain.Level_09_Terrain:PersistentLevel.L09_Landscape.Texture2D_5 is unknown [raw type 0] which is not supported.
Any idea how it happened, what it actually means and how to fix it?
I have that on multiple of my landscapes!