I was playing with my friend and everything was fine then when the sserver restarted it kept saying for my friend"you do not have permission" even though i invited him to my island and it was working fine before the restart.
This is a bug that append some time, you have to kick him from the island and invite again. Be careful because kicking someone from the island will reset the inventory. We will need the name of your friend to remove the items and put on a chest.
