If you want to drag-and-drop a potion from the shortcut bar on the bottom of the screen back in the inventory, the potion just disappears. WHY!? I just want to split it up because I want to give my other partymembers some potions. But I fukkin can`t. WTF ?

Press 'I' and you can open your inventory. Then in your inventory you can right click in your inventory and click: split. Then right click to send it to your party member. You can remove it from the hotbar, if you want. The thing is, this bar is just reflecting what is in your inventory.