BorgBase - This service allows sftp connections only #10606

I have the free / trial plan on BorgBase.com. I was previously able to import a local backup repo to this BorgBase account, using the instructions found at https://docs.borgbase.com/setup/import. This would appear to confirm that I've set up the ssh keys, permissions and so on to my account correctly, and am using the correct URL (which looks like ssh://<repo-slug>@repo-slug>.repo.borgbase.com/./repo).

My problem is that I cannot create any new archives on the imported repo. Trying to do so gives the error This service allows sftp connections only.

I tried creating an archive with:

$ BORG_REPO="ssh://<repo-slug>@repo-slug>.repo.borgbase.com/./repo" borg create ::TEST ~/Documents
$ borg create "ssh://<repo-slug>@repo-slug>.repo.borgbase.com/./repo"::TEST ~/Documents

Both commands gave the same error. It's not just create; other commands fail similarly:

$ borg version <repo-slug>@repo-slug>.repo.borgbase.com/./repo
/usr/lib/python3/dist-packages/borg/remote.py:1023: ResourceWarning: unclosed file <_io.FileIO name=7 mode='rb' closefd=True>
Got unexpected RPC data format from server:
This service allows sftp connections only.

By running borg version . I could at least get the version number of the local client. I tried running the commands with two different versions (1.4.0 and 1.4.4); both gave the same error. On the BorgBase side, my repo is set to use borg version 1.4.x.

Am I missing something in my borg(backup) configuration, or the way I call the command? Or is this an issue specific to BorgBackup.com?