A very warm it's a pleasure finding you here today.
There's be a tremendous amount of soft shell problems in chickens. I have actually lost count of how many I have heard of. My last 3 chickens had the same problems
I was so sad to read that you lost your duck. At least she was cared for while she was with you.
People have so much knowledge on BYC, about many species. You will find threads on all sorts of subjects.
I'm so sorry for the loss of your duck. Sometimes we can take the best care of our poultry and we may never know what happened.
You mentioned she laid some soft-shelled eggs. If you find your Rouen is laying those, it may need a bit more calcium, which is easily given with oyster shells.