Hi,
a bit different, but same error:
Since my storagebox are full, I added an additional backup site to an s3-bucket.
After starting backup, it fails shortly with the error FrenchyBear mentioned: "BoxError: Old backup not found"
For the new-s3-site i'm using rsync with encryption, the bucket is new, so there is nothing for cloudron to reference to.
The problem was with 9.0.12 and is also there, after installation of new version 9.0.13.
So my account is new, it seems I can not add a screenshot.