I am by no means an expert on Booleans in Carrara - what I do know is that Booleans should be avoided at all costs - caus' they just don't work that well. I think the problem you display is because one object is a "solid" object and the other is just a curved plane. I've run into this problem a few times myself. The only solution I've found is to go into the vertex modeller - the holes are made - you just have to manually disconnect the verticies that are messed up.