Alessandro, that added chelating agent is not only a good idea, it is common practice, see Fuji's E6 formulas posted here. I read somewhere, that these reexposure agents suffer from sudden death syndrome, a bit like Xtol, and that's apparently what hit Francesco here. While it may be somewhat difficult to get SnCl2 as raw component, Sodium Dithionite is easy to obtain and works just as well.