Ducks do tend to keep a moist environment if they have access to water inside the coop and their poop is more watery. Do you keep water inside the coop overnight? If they have water, they're going to use it and oftentimes make a "fun experience" with it. If everyone is wet, they're possibly getting splashed or there's not enough ventilation. Some materials allow for condensation and depending on your coop material, maybe that's an issue?
My apologies. Edited for clarification. My words didn't convey what I meant