バックアップ失敗、一時バックアップファイルがbackups/defaultディレクトリに移動された際

[pfaffman (Jay Pfaffman)]

@ckshen

does this issue have anything to do with the fact that I previously created the /var/discourse/shared/standalone/backups/default/ folder (outside the container) myself when I used command line to restore a backup on a new instance

Probably. Try deleting it.