Marron glacé / Candied chestnuts breaking up. How to avoid this?
This is a guess, but I'm assuming the sugar concentration in the syrup that you're using is way too high.  After a few days, the water evaporates off and the sugar is left in a crystalline form inside the chestnuts.

This makes them very brittle at which point you see the breaking.

If this is the case, I can recommend two things:


Reduce the rate at which the sugar in your confections crystallizes; left in the open air, you're going to lose moisture which is part of the problem.
Increase the water ratio.  I'd start with a simple syrup (1 part water to 1 part sugar) and move up and down from there.  Of course, your going to sacrifice some sweetness as a result, and you don't want too much water, as you'll end up making the chestnuts too soft.  You'll have to adjust the balance to your tastes/tolerances.