Multiplayer broke

After updating to 1.12, when I joined my world a message popped up saying that multiplayer has to be diabled for that world if I have a custom skin on. After that, no one can join me in ANY world, not just the one I joined. I can join others on the same world even if they have custom skins but no one can join me in any multiplayer world :/ They get the "Unable to connect to world" message