I have a Minecraft server with my friends and multiple people (including myself) have stopped taking damage altogether. I am the only op, the difficulty is Normal and we are all in survival. Arrows bounce off of us, zombies do no damage, nor do creepers or any other mob I've tried. The void still kills us and /kill works. potions of instant harming do nothing and I do not hear a hurt or shield block sound when mobs attack me.