I am trying to import a 3D object from Cinema4D into Three.js using GLTFLoader. However, this object is completely black with an AmbientLight. Now I have found out that it is due to the material used. If I now nest the GLTFLoader in a RGBELoader, my 3D object gets more correct lighting (see the screenshots).

1. Why does my 3D object not react to AmbientLight, but to the RGBELoader? What is the technical difference between the light sources?
2. Can I initialize a kind of neutral RGBELoader light source in Three.js? A white image? Use a different Three.js method? Kind of AmbientLight only that it reacts to my material?

Attachments:

new RGBELoader()
    .load( 'royal_esplanade_1k.hdr', function ( texture ) {
        texture.mapping = THREE.EquirectangularReflectionMapping;
        scene.background = texture;
        scene.environment = texture;
        render();

        // model
        const loader = new GLTFLoader();
        loader.load( 'Cube2.glb', function ( gltf ) {
            scene.add( gltf.scene );
            render();
        } );
    } );

const light = new THREE.AmbientLight( 0xffffff, 1 ); // soft white light
scene.add( light );

scene.background = new THREE.Color( 0x808080 );

const loader = new GLTFLoader();
loader.load( 'Cube2.glb', function ( gltf ) {
    scene.add( gltf.scene );
    render();
} );