Question Can't assign custom Material to Terrain anymore

I'm currently working on a custom Terrain shader and got it working. Now I tried to expand my Terrain but ran into a problem when creating new Terrain and assigning my custom material to it. The terrain won't render at all and I get the following error:

Instancing: Unhandled property 'unity_ObjectToWorldArray'. Values are undefined.
UnityEngine.GUIUtility: ProcessEvent (int,intptr,bool&)

At the same time I have my "working" Terrain with the same Material Assigned to it, which renders perfectly fine in the same Scene.

Even started a whole new project reproducing the error in a simpler way: Create new terrain -> assign any default material to it (other than default terrain materials) -> same problem and same error message.

Has anyone else had that problem? It drives me crazy and can't figure out what the problem is. Plus I'm 100% sure that assigning any standard material, even if it is not a unity terrain material, worked before Yesterday. I didn't change anything in my Terrain Tools Package or Unity Version in the meantime.

Currently using:
Terrain Tools Version 4.0.5 - February 13, 2023
Unity 2021.3.12f1
Built - in 3D template Render Pipeline

 

Found the difference myself: When I create the new terrain with the terrain toolbox, the custom terrain shader messes up the whole scene and doesn't work at all. But if I create the terrain in hirarchy - via right click - create terrain, everything works fine.