I'm not really familiar with the lastest version of the game, maybe it is a client glitch that prevent you from dragging your weapon because you are already holding something?
i think there is a command for "droptopack" or something similar, try to do that on yourself.
furthermore you ould check if:
pack full? max item reached? out of range? access denied? trade pending? --edit: also try from an external character, aka moving the weapon of someone else from his paperoll to his pack.
many things can get into conflict, try with a fresh char, the first thing you do is equip, unequip, equip, unequip, if this works, move a piece of cloth from your paperdoll, move it to your pack, wear it again, and repeat first step.