Black squares on render

[Miniminer]

Hi,

I'm a rather "newbie" in the world of Mental Ray and I had to render my first image sequence yesterday. While putting all the images together I saw on a few renders there are some black squares (the size of the "calculating square").

I can't find any pattern on which image it occurred, and it's on a different place, but in the same area. (it's a sequence of 283 images and it appears 4 times)

While searching on Google I found out some other users already had this problem but on Maya, not on 3dsm...

I will try tomorrow to re-render this particular images and see if the square appears again, I will edit my post with the result.

My (very rough) specs:
1280 x 720
Sunlight
FG
Motion blur
& glare

(if you need more specs let me know)

Thanks for your time.


edit: after the re-render the frames are okay