Sur reddit, certains disent qu'insister comme un gros bourrin permet de reprendre les télechargements.
https://www.reddit.com/r/NintendoSwitch/comments/nx5ywq/psa_if_you_are_getting_error_code_21231502_when/
"I've managed to update two games just by pressing okay when the error pops up then continue on the download screen, takes 5 or 6 tries but eventually it goes through"
"I got the error earlier, all i did was click on whatever failed a few times, eventually the screen with buttons including "pause download" popped up.
simply wait on this screen and the download will resume, then you can close the screen."
C'est un peu stupide mais c'est à tenter si des gens sont pressé de télécharger un truc 