Larian Studios Forums Divinity: Original Sin 2 - The Modding Dungeon Divinity - Original Sin 2 - Modding Troubleshooting Editor crashes when creating new terrain

A 1x1 terrain patch is already quite big since DOS EE (I think 128mx128m in DOS EE, not sure about DOS2). So 256x256 is probably gigantic. Try starting with a 1x1 to get a feeling of how big a patch is, and then scale accordingly. My guess is that you're running out of memory.