i have the same the issue like you but i finally know how to fix this. If you have enabled Extra Equip mod on your server, you have to simply drop your backpack and unpack the box. This works for me. I discovered this when i tried to unpack my box (while equipping my backpack) it wouldnt work even tho there was a huge space, but when i dropped my backpack I finally can unpack the box. I dont know if this REALLY works for everyone but atleast i give my answer.