Don't use msg.sender for view functions,

await nftContract.getTokenUnlockableContent(0)

Sets msg.sender to 0 by default. You can specify a from parameter if you want when calling a view functions (which remix does, thats why it works there) but the best option is always to have an address parameter in your function and not rely on msg.sender

Don't use msg.sender for view functions,

await nftContract.getTokenUnlockableContent(0)

Sets msg.sender to 0 by default. You can specify a from parameter if you want when calling a view functions (which remix does, thats why it works there) but the best option is always to have an address parameter in your function and not rely on msg.sender

EDIT : I also noticed you're making a check for msg.sender == ownerOf(tokenId); if you're using that hoping that it would prevent non-token-owners from calling thé function, it doesnt work. And if you want data to remain private, don't store it in a smart contract in the first place.