If you are in the situation that backups created before the upgrade are not showing or cannot be restored, run on the backup server
apt update && apt install ecp-core
v12-upgrade fix-backup-permissions /backups
Where /backups
is the path that your backups are stored.
This won't do anything but it will output some recommended commands to fix the ownerships and add users where appropriate. If you have any questions or problems please send a support ticket.