This may be related to shadow algorithm. Try to add:

renderer.shadowMap.type = THREE.PCFSoftShadowMap;

or

renderer.shadowMap.type = THREE.PCFShadowMap;

And increase your light source shadowmap size:

<lightSource>.shadow.mapSize.width = 2048;
<lightSource>.shadow.mapSize.height = 2048;

closed helped me

<lightSource>.shadow.bias = -0.0005;