It could be because you used MGS header, but like you said it's month old.
But I doubt the cia is the cause, users are not banned when playing, but when exiting the game and trying to reconnect.
You can play online, nintendo ban the user, the user can still continue to play online (there's no expulsion).
If you say that you redeem a code, it means you didn't connect online before redeeming it, there was no detection possible to ban you, so I don't see why you would be banned.
Not banned > Play online > detection > ban > continue playing online > stop playing > try to reconnect : fail
Not banned > redeem > ... you are not ban yet, so why the error?
What is this redeem menu? where is it located? do you have to be online first? disconnect then reconnect ?
offline > redeem
or
offline > online > redeem