I (more or less) found the problem. I discovered that using a render area it didn't crash. Then I just moved the render area around the picture until it crashed. Ok, I had found the problematic area. There were 3 objects in that area. I deleted all and the problem disapeared. Then, put the objects back again, one by one and found it crashed with any of 2 of them. I replaced the materials in these (which were just mixed material for water in one and mixed material sand in the other) and found that these two materials had been corrupted somehow. Both of them! They were used in other previous pictures without any problem at all. Weird! I built the materials again and it worked fine.
What a weird thing!!