Those are large roundworms, looks like an infestation at that. I recommend using valbazen cattle/sheep wormer. Dosage is 1/2cc orally undiluted for standard size birds, 1/4cc for smaller birds. Redose again in 10 days. There's a 14 day withdrawal after last dosing.
Valbazen slowly kills worms over several days unlike other wormers. No need to worry about toxic dead worm overload that other wormers will cause when there's a worm infestation. If your feed store doesnt carry valbazen, it can be purchased from Jefferslivestock.com or call them. They ship quick.
Yes those are roundworms - I had a similar issue this past summer. Found a bunch of dried up ones in the manure box where I use Sweet PDZ to keep it dry and smelling better.
After you treat with the dewormer like Dawg suggests - you may want to consider using some natural dewormer products in the future to keep the worm loads reduced. Some of them are to feed your chickens whole pumpkins with seeds - just leave the pumpkin somewhere so that it can rot away and they will eat it over time. Another one is garlic - sprinkle garlic powder on some scrambled eggs or put it on their feed. Another is to use soap in their waterer - just a drop or two of dish soap like Dawn will work to help create an environment where the worms won't be able to live. Lastly put ACV (apple cider vinegar) with the mother in it (Braggs or Walmart sales Heinz) - into their water. Just add a tablespoon per gallon.
You might also consider feeding them fermented feed to help them naturally stay healthy and ward off parasites. Hope they are back to feeling like themselves soon.
I just noticed that you live in Germany and may not have access to the wormers mentioned in this thread. Hopefully you should be able to get some Flubenvet there. That's a good wormer and will take care of large roundworms as well as a few other types of worms.