Unable to use external storage

I got an sftp external storage configured. I verified it's working by copying data to it trough nextcloud. However, the backup app shows "No external storage available.

My external storage setup:

Backup app:

(note the 'unable to fetch external storage' message)

I'm running nc v23.0.0 with backup app v1.0.0 through the official docker image with MySQL as database.
I cannot see any error messages in nextcloud.log or docker logs (includes apache error and access logs).

EDIT:
Upon further inspection I found a failed GET request to https://redacted.tld/ocs/v2.php/apps/backup/external. It's an error 400 with a body

{
   "ocs":{
      "meta":{
         "status":"failure",
         "statuscode":400,
         "message":"no authentication parameters specified"
      },
      "data":[]
      }
}

Start by reproducing the failed GET request to /ocs/v2.php/apps/backup/external with the reported Nextcloud 23.0.0, Backup 1.0.0, Docker, and MySQL setup. Inspect the request authentication and the nextcloud.log and Docker logs; the issue is done when the backup app can fetch and use the configured SFTP external storage instead of returning HTTP 400.