Sounds like you're using some mod that places your world not on the default overwolrd surface, usually called "nauvis." There should already be support for New Game+, which does this. Can you send me a mod list, particularly any that might add or modify surfaces? Also, if you can run the following command, it will list all the surfaces on your server:
/c local out = {} for k in pairs(game.surfaces) do table.insert(out, k) end game.print(table.concat(out, ","))