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I have just encountered a game breaking bug where quest "Missing Person" does not start.

This happens if you choose not to go with Yennefer after the quest "Echoes of the Past" in Skellige.

There are 2 solutions to this:

1: Stay until the portal through which Yen goes closes.

2: If you like me rushed off in some direction to do which ever quest you had in mind and didnt stay for that, then this is what you do. Go back to the place where Yen lifted the tree. Move around a little bit around the area. My suggestion is recreate your steps and walk the same path you walked to that location and the quest "Missing Person" should appear after which you can continue your main game on your marry way.

I do not know if this will work for everyone, but it did for me.