Thank you for the link. It gave me the confirmation that it was safe to remove this data from the backup. So I did, then repackaged the backup, and restored it with Yunohost as usual and this time it worked.
I haven’t noticed anything wrong with Nextcloud and the Memories app after the restoration.
I don’t know if the problem comes from how the Nextcloud app for Yunohost is set up, or if it’s the backup/restore system from Yunohost itself that caused this issue, but I managed to fix it by myself for now, so I’ll set this topic to solved.