This is a version mismatch problem, we’ve had some previous questions with the same issue:
Error: Unsupported babylon environment map version “2” - Questions - Babylon.js (babylonjs.com)
.env conversion from .hdr throws an error - Questions - Babylon.js (babylonjs.com)
Sandbox now uses preview version 5.0, and your scene is probably using 4.2 
Updating the scene to 5.0 is the recommended approach, since it’s backwards compatible and you get some improvements for free!