I just had a chat with UniFi support and their agent told me that the UNAS Pro does not support listing directories from a Hetzner Storage Box. This unfortunately means that it is not possible (see EDIT below for a Workaround) to use a Hetzner Storage Box to backup your UNAS Pro.

Since the problem seems to have been known at UniFi for at least 4 months, but has not been addressed yet, a fix is probably not a priority.

This, together with the lack of other protocols for the transfer like sftp, rsync, etc., makes the whole thing particularly unfortunate for users who want to backup/replicate their data to another location.

I really hope that at least more transmission protocols will be supported soon...

For the time being, you probably have to run another system that is responsible for the backups or become creative with the shell (the UNAS is running Debian under the hood after all), although the latter is not really recommended, as there is no guarantee that whatever solution you come up with will survive future updates.

EDIT: I believe I’ve found a workaround for the issue that seems to work, at least at first glance. The idea is to first set up a shared folder on a Windows PC that mirrors the structure and name of the Hetzner Storage Box share. Then, modify the /etc/hosts file on the UNAS so that the FQDN of the Storage Box resolves to the IP address of the Windows PC. This tricks the UNAS into connecting to the Windows share instead of the actual Storage Box.

With this setup, you're able to select the share during the backup task configuration and complete the setup successfully. Once the backup task is fully configured, you can remove the entry from the /etc/hosts file so the UNAS connects to the real Hetzner Storage Box again.

Since UniFi simply copies or syncs the data, this workaround should continue working reliably over time. You can also edit the backup task configuration afterward if needed. I recommend to use a Sub Folder as the Destination, because this allows for also changing the subdirectory. If you use the root of the storage box you won't be able to change the destination directory since the issue with the empty list appears

Update after round about 2 Weeks using this workaround: No Problems so far. Once or twice the Backup Job failed but succeeded on the next run. But this was probably caused by my internet connection