EDIT: I just found out this has nothing to do with the "An Island of Eels" quest. I just dit a the contract "The Stolen Purse" where I have to kill someone. The game doesn't register that I killed the target aswell.
After killing Wigmund the game still wants me to kill him, but because he is already dead I am stuck. Tried going back to a previous save but after killing Wigmund again the game still didn't register him as dead.