Hello and sorry for the late reply, but in the end we managed to find out what the problem was. I don't have an screenshot though, but apparently there's a three day cooldown from the moment you add a new friend and the moment you can send them stuff. I was able to send the plate to a friend in common and that person sent the plate to the person I wanted to send the plate to. But maybe you should fix the translation, something like "you haven't been friends long enough" or something more informative.