Meshes are lit black towards edges

I can’t figure out why, and I don’t really know how to describe it, but this is happening:

In the pictures below, there are three meshes on top of each other. They align exactly, and the surface is completely flat. (Yes, I am aware the wallpaper is upside down)

Without lighting:

With lighting:

Why is this happening? How can I get rid of this?

EDIT

Another example:

alt text

This is likely caused by the smoothing on your mesh…
Select your mesh asset in the project view, and then in the inspector under “Normals and Tangents”, if “Normals” is set to import, change it to “Calculate”, mess around with the smoothing angle slider and click Apply. Hope this helps!