I have a hen who has the same problem every once in a while. I'm not sure what causes it, but I clean her butt feathers off everytime it happens. Take a bucket of warm water, plenty of paper towels, and some scissors. If you can't massage the poop off her feathers with a warm paper towel, you can clip the poop off the feathers. Don't clip the feathers too far back, but enough to get some of the poop off. I prefer to wear gloves while doing this, it can get messy. You will also likely need a person to hold the bird for you too.
You may get a lot of varying answers on this, but I have a few like this in a big flock. They eat the same thing, but more of my ameraucans have it than some other breeds. Others on different threads I read have the same problem. Some will tell you they have worms, but I think it's just a problem some hens have, and I just trim the poo and feathers around their vents and that solves the problem. You can wash them off in a pan of soapy water too. My chickens have been on regular feed and fermented feed for 8 months, now back on dry feed, but it makes no difference. They don't have a big worm load either.