I was following https://docs.mailcow.email/backup_restore/b_n_r-backup/ today, running command MAILCOW_BACKUP_LOCATION=/opt/backup /opt/mailcow-dockerized/helper-scripts/backup_and_restore.sh backup vmail, and after completion discovered all space used on disk. I didn´t notice any error message, but question that the resulting backup is complete.
Are you doing any error checking during backup?