Hello everybody ,
This error code often appears when you want to equip a cosmetic for the head. The reason is that you can not equip a cosmetic for the head if you wear gear that protects the face and head.
So,if for example you wear the "Snowman Head" you will not be able to wear any cosmetic for the face or head.

Screen here: http://imagesia.com/combat-arms-170711-173849-00_1f9a4