Petra...my guess is that they are carriers of salmonella or E-coli, either of which can cause the babies to die in the shell or die young. As carriers...mom and dad may act just fine. Remember though, birds do pretend to feel okay, even when they aren't as a natural defence against predators.
If they birds were in my care, I would treat them both with baytril or a baytril generic, for 14 days.
I can't think of a natural remedy that will cure them.