I'm having this very same problem. I ordered three breeds from a hatchery and I have lost all of the Sussex, then all the cuckoo Marans pullets and one cockeral, and two rocks. I have one rock that started getting the very same symptoms again after about two weeks of no incident. I'm ready to give on chickens all together after losing this many birds to some unidentifiable disease.
The symptoms start when they stumble around and then they just lay down. They still eat and drink, just not as much. The vent is really nasty by this point but I don't see any blood or worms in the stool. Within 24-48 hours of the first signs they have passed.
I've isolated and moved the area where they stay and given ACV, vitamins, garlic, and oregano all in various quantities and nothing helps. They get to free range all day and I give them a 22% protein multi-flock food. I have turkeys and ducks that I keep separate.