Hard to say; I'm sorry you've had no suggestions. I think I would start with a bath in mildly soapy water for a few minutes, then get her completely dry so you can see it better. Dawn dish soap would be good, just a little. Check the water for any sign of tiny bugs. See if you see a wound. You could try a thin smear of neosporin or an antifungal cream such as what is sold for athlete's foot. It's possible, even, that you are smelling normal chicken smell. It certainly is normal for them to "groom," or preen, which involves cleaning off their feathers and skin.
