The path to your steam library is not allowed to contain a . (dot). Unfonrtunately this is the default if you did not changed your library path. The default on a steam deck is /home/deck/.local/share/Steam/steamapps/common/RollerCoaster Tycoon Deluxe/ If you add another folder as library e.g. /home/deck/SteamLibrary and move the game installation to it. After that saving and loading works as expected.