Copying savegame between the accounts

Nothing can be transferred between different accounts.

Can you try to start the game with a different Gamertag Account?

Maybe a world in a savegame became corrupted for that Gamertag Profile, and the game can't load the corrupted world.

If the game starts with a newly created profile, you should try to backup your savegames except one to an external USB drive, disconnect the drive, and try to start the game.

If it doesn't work, delete that savegame, and copy a different one over. If that works, copy one savegame over again, until the game gives an error again, or if all other working savegames are copied over.

The Xbox One has a specially reserved storage space to keep a local copy of the cloud savegames stored online.

That storage space is only used to keep a local copy of your savegames to speed up loading, or to store savegames temporarily when your console was not connected online.

If you clear that save game storage, this storage is cleared.

If the console was connected online, your savegames are already synced to the cloud, and will be re-downloaded the next time you start a game.

If there are unsynchronized savegames on your console, because the console was offline, your savegame for that offline play will be lost.

it depends on your setup if you can use your stuff offline. in general every gamertag and every savegame can be used offline, in case of savegame use your hdd storage instead of cloud. if you used cloud, you can still copy your savegames to hdd as there
is a local cache for your savegames in cloud settings. games can be used offline under following condition: it does not explicit require internet and it was purchased on your own account.

You can backup your savegames by using the cloud storage or by copying/moving them to a configurated USB-device.

Your stats are stored in your profile. Checkpoints are stored in your savegame.