Landscape Error: Texture is unknown which is not supported ? How to fix?

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!

I can’t speak to the landscape side of things, but I noticed this same “Unknown raw type 0” error for the first time the other day, except for me it is with a curve atlas, and just comes up occasionally. It’s on 4.25. I gave it a resave and re-saved the mats that use it, and I don’t think I’ve seen it since, but it’s only been a day.