Puzzling.
I assume you are triggering a backup manually to test?
sudo ~/mailinabox/management/backup.py
Otherwise it waits to the 3am hour I believe.
For some reason, though set the UI does not show anything in the fields other than S3 path (at least on my 0.51 box). That I found confusing.
My apologies if my suggestions are too basic. It is hard to know what is going on here.
I might try Scaleway myself. I see they offer 75GB free object storage.