Catalin,
I know you are a vet. Can you test them for coccidiosis? Bresse seem to be a little more vulnerable to it and that is the right age. Remember they don't always show signs like bloody poop.
I did test my flock earlier this spring, before I started hatching & brooding. They do have a minimal level of coccidia. These birds have not been in contact with my other birds, yet. Would a coccidial infection cause these sorts of symptoms?