When I exited from below Caed Nua via the ladder that leads to the garden I tried to re-enter the cellar the same way, but all I got was a blank map overlay with the title "Paths of Old Nua".

Hey Gusmonkey,

 

The map is starting in the wrong position. It seems to default you to the bottom of the map which has not be revealed yet. If you scroll up to the top you can see what you have uncovered and travel to the different levels. I'm putting this issue into our database and we will look to fix it in an upcoming patch.

 

Keep of the good work citizen

I had the same problem.