Your frustration is understandable, Nicola.

But don't give up, please!

What intrigues me is the fact that the render can stop randomly even with no light and that it stops without any warning as if it considered it had finished its job...

What kind of things can change from one render to another when the scene is not changed? What could explain that sometimes it works and another time it doesn't? Something related to P6 (necessarily) that changes while the rest is unchanged... I'm dry. I need to think more.

Definitely, contact EF.