Thank you Otakar, Jo, Chronor for your kind comments.
I have done a close up quick render and the problem is still visible : some white pots at the boundary between the lake and the terrain which is a heightfield (generate) with Alpine fractal shader.
As to the crashes I experimented, I have checked the "show as bounding boxes" in the preview window setting so that it is quite fluid. The crashes occur during the population process of the grass as long as I use a painted shader for it. Is this a bug or not, I don't know.