Error when requesting vault cooking for a directory in pending state

I just tried to trigger a vault download by clicking on "Download" on https://archive.softwareheritage.org/browse/origin/directory/?origin_url=https://github.com/ProgVal/Limnoria

This sent a request to https://archive.softwareheritage.org/api/1/vault/flat/swh:1:dir:a5ee0efecc576af1404ca89e4b8a0eb938b4a279/ which answered with code 200 and this body:

{"fetch_url":"https://archive.softwareheritage.org/api/1/vault/flat/swh:1:dir:a5ee0efecc576af1404ca89e4b8a0eb938b4a279/raw/","progress_message":"Processing...","id":415260436,"status":"pending","swhid":"swh:1:dir:a5ee0efecc576af1404ca89e4b8a0eb938b4a279"}

The UI then showed me this message:

Archive cooking service is currently experiencing issues.
Please try again later.

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Migrated from T4698 (view on Phabricator)