When attempting to enter an open doorway in the ruins, it claims it is locked. The lock prevents entrance to the starting island from the palace island. I think the cause is that the other side room has pressure plates that lock the door. The door was locked when last in the ruins, but when I traveled via BFB I was reconnected to the palace island and traveled the opposite way. I assume it still counts the door locked from the other side which is now impossible to access.
Lock a door via pressure plate in a ruins that leads to another island, then travel to the other island through other means (such as BFB) and try to enter the door that was locked through the opposite side.