the reason for this is because it's in the shield slot, if you was the have a shield in that slot and you use the slot it hold the shield up to your face. so the server thinks that your backpack is a shield. allthough it's just an item that increases Weight stat. so your character tries to use the "shield" backpack but it cannot as it's auto used. it may be fixable maybe not, i wouldnt worry about this..

Because...
If you have a shield and a sword, you can hit and block.
if you have a shielf and a Pike, you can only use the pike as its a two handed weapon.

if you have a backpack and a sword, you can hit, and you try to block but fail because your backpack.
if you have a backpack and a pike, you can throw / use the pike as normal as its two handed.

it's just an animation dont worry about it. Treat the backpack as a shield allthough it won't protect you much.

maybe a way to fix this problem could be to Change the Backpack from it's own slot and have it as an Add-on / Skin type thing for your Chest armor. that you can remove / re-add the backpack to the chest peace, so that the backpack and the chest peace can be changed and does not take up shield slot.. like Scope's and ammo.