All rigidbodies fall through terrain

Any gameobject with a rigidbody component will fall through the terrain. The Terrain has a mesh collider and terrain collider but they still fall through the terrain. I tested a cube with a rigidbody and had it fall on a plane, and it didn’t fall through. So there is something wrong with my terrain but i don’t know what. The only thing that doesn’t fall through the terrain is the First person FPSController. What can i do?

Your Gameobjects need collider and rigidbodys.
Make sure your colliders are not trigger colliders.

This isn’t enough. Here is my terrain:

And here is my Loot:

The loot falls through the terrain. Why?