I'm very sorry to hear about your duck
@Fabfur . To let us help YOU, could you provide us with some additional information regarding the bird?
How old is she? What is she being fed? Do you have any pictures of her setup? Have you picked her up and inspected her legs to ensure they bend at the joints correctly? Is there any swelling, on the leg? If unsure you can use a caliper to measure compared to the good leg. Any discoloration, lesions, or warm spots? Is she currently eating, and drinking? What kind of substrate is she being kept on? Did the leg problems appear slowly over a few days, or overnight?
The video will be helpful. I see azygous was kind enough to explain how to post a video.