Released on October 14 2016. It affects the following platforms: PC, Mac.
Hotfix 10/14/16 – Plunder Skull TradingEdit
We just completed a hotfix on Xbox One and PlayStation 4 that now restricts you from trading Plunder Skulls - items gained from bosses during the Witches Festival - within members of your group. This functionality is already on PC with Update 12, and while there was no real harm in trading with each other, doing so was causing performance issues including causing you and other players around you to disconnect from the game.
Even though you will no longer be able to trade Plunder Skulls, you can still freely trade any items you receive from the Plunder Skulls.
You do not need to download a patch for this change to take effect.