I'm using the CloudFlare service (there's a plugin for that, and it's supported by W3 Total Cache too) which is in essence a reverse proxy that intercepts requests for the domain (at DNS level) and then, after scrutinizing the originating IP and request, passes on to / from the real webhost. It's a fundamental security thing (and as a bonus you get some quasi-CDN caching benefits) and it's serving me very, very well.
However, since Online Backup for WP picks up the backup packages, that is very likely why the online backups (both manual and scheduled) are consistently failing, with the following error message:
The backup failed: The server attempted to retrieve the data, but received junk from your blog. This can happen if your blog is not accessible over the internet. Otherwise, you may have a third-party plugin installed that is changing the backup data as the server tries to receive it. Please contact support if this is the case so we may improve compatibility.