Gloss black can play some tricks on the eye, so it is a little difficult to tell what is going on in the photo but I can guess.
I believe you've experienced the icky soup skin effect. You poured a layer of thinned paint. The surface, being exposed to the air, dried, while everything below the surface was protected. As the paint dries, these molecule chains link up together, forming a skin. As the skin continues to cure and lose water, it shrinks, and as it shrinks, it tightens, and since the only thing beneath it is liquid paint, it cracks. In the opposite direction, the wet paint beneath is still trying to evaporate and push out. You see the same effect in mud as it dries out.
If you tried this before with no problem, it might be because you were working in a different humidity or a different temperature, or you just didn't apply it in such a thick pool when you tested.
Any time you deal with a product that involves something that must evaporate off for it to cure, whether it's water, or alcohol, or paint thinner, the product must be applied in thin coats or you'll get this sort of cracking.
If you want to fill in a section, you need to use a product that cures through a chemical reaction alone, like plaster, or epoxy, or has enough solids mixed in so that the water is forced to diffuse through slowly, like wood putty or jointing compound. Even then, it'll still crack if dried too quickly, so it can relieve stresses built up as it shrinks.