Sleep well, Nicola. I wish I could have been more efficient. :S

Okay. This scene will be a good test scene.

Here's what I propose you: delete all lights. Then render. Does it work completely?

If yes, then create a new light, an infinite for example. Render. Dooes it render completely?

Once again, if yes, add a second light and so on.

What are the results?

Also, while doing these tests, could you open the task manager and use its graph mode. Look at the cpu and kernel levels (you can activate it via the option of display -> kernel time, I think). When the render fails, do you see a huge increasement in the kernel use? Also, what is the amount of memory used by the Poser.exe process when it fails?