Unable to Open Loot-Filled Pumpkin

While in many cases, looting issues can be caused by add ons or a corrupted interface, in most situations the inability to loot items would be due to unique items. If you have an item in your inventory, you would be unable to loot another from a container or remove it from the mail box. For the Loot-Filled Pumpkins specifically, check your inventory for masks. Use, sell or destroy the one you already have in your inventory and this should allow you to then loot the new one. If you are unable to open it at all, then you will want to disable add ons/overlays. If still no luck, resetting corrupted files may be what is needed: https://us.battle.net/support/en/article/7549