So backups by default would end up in /var/backups unless otherwise configured. It is strongly recommended to use some backup storage, which is not on the same disk. It may be useful to checkout https://docs.cloudron.io/backups/ for more detail.
Regarding the Nextcloud instance as such and the error you are facing, I am not sure how your setup now looks like, but is /media/aeternal.cloud some volume you have moved Nextcloud to and if so, is this properly mounted?