Look, I don't want to insist if you say it works, but before reporting the issue I have tried it on 3 different GDevelop starter projects that, as you know, are barebones so I really don't know what's happening here. And I didn't add any custom extension or JS.
Here is a screen from a few moments ago. I've started a new project with 3D shooter as base this time, I've only removed completely the skybox layer effect and added the env map extension, put default sky images just to check if it shows something but it still gives errors and the sky is black just like the other 4 times/projects. Switched to JPG images and nothing changed. Updated GD to latest version and still not working. (just as a note, I have already tried multiple times switching on/off all options, checked if at least environment maps on models works, using different cubemaps even processed on the web tool you suggested. Nothing makes a difference.)
