I just had the same issue. It happened to me after dying to a wisp in the dungeon under the hanging tree - my companion disappeared, white circle instead of his portrait. Strangely, I could see the green circle that's usually underneath him at the start of the area, I clicked it and tried to move it, but it then just flew off the bottom of the screen.
I had a save from 5 minutes before, I still have the issue if I load that one - completely unable to change area now as the party isn't all there. Time to restart