Looks like there is an Unity SDK implementation error when using GetLeaderboard (Server or Client API).
Using the new ProfileConstraints with ShowLocations = true returns a list of PlayerLocation objects inside the PlayerProfile property of each entry.
For some reason the PlayerProfile definition in PlayFabClientModel.cs and PlayFabServerModel.cs expect a:
public Dictionary<string,PlayerLocation> Locations;
which of course cannot be serialised then.
Changing this to:
public List<PlayerLocation> Locations;
makes everything working.