Provided the mesh is properly welded and the black cracks are not actually cracks in the mesh, try the morphbrush and smooth that "cracked" areas a bit.

Looks like the normals are messed up because the mesh intersected itself while draping.

And yes, this can happen more easily with high res meshes than low res meshes.

 

First try "Normals_Forward" in the material settings.

If the cracks go away, then it's a normals problem and smoothing with the morphbrush should fix it, as it will re-arrange the normals.