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the scene is lit up using three spots placed 120º apart
When using images textures that are not meant to be color information (such as the specular and normal maps of your material), you have to set their color space from Color to Non-Color Data :
Without that, they won't behave as they should and might produce artifacts.
In Node Editor window: Try adding Texture Coordinate and connect UV node into vector nodes of all the Image Textures you have. (Node Editor: Shift+A>Input>Texture Coordinate)
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