Problem with collisions: Player and Terrain

Hi everyone!

I’m making my first 3D game in Unity (I don’t have much experience right now but I want to learn). I have some problems with collisions in my game. I’m using a terrain object from Unity and a ThirdPersonController from Standard Assets (Ethan character with rigidbody and capsule collider attached to it). My terrain object has some parts which are completely flat and some which are sculpted to have lower areas (to make little pools with water). It has a terrain collider attached. When my player jumps and falls down on something which isn’t flat then the problem is visible like on the example image below – my player falls through the terrain and then freezes in some point. There's no problem with movement and jumping on a flat surface.

I don’t know how to fix this – but I know it’s not acceptable at all. I've tried to change some values in player's capsule collider to make it bigger or smaller but without success - the problem still exists. Could you please tell me how can I fix this problem or give some other advice how to make it disappear? I’m really open to all suggestions.

Thanks!