Duplicati cannot backup to SSH on Debian after remote server update (Algorithm negotiation fail)

Question

This issue may be relevant for other Duplicati users:

Duplicati is configured to backup via SSH on a Debian webserver. After this webserver was updated, the backup failed with the error message "Algorithm negotiation fail".

SFTP via FileZilla is still possible, using the same credentials.

Answer 1

The problem is that "insecure" algorithms were disabled for SSH (in 2014/15): https://stackoverflow.com/a/26607212/336311

One possible solution may be to re-enable these algorithms (see link above). But if you cannot (or do not want to) mess around with the SSH configuration, there is a more simple solution:

1. Download psftp.exe form the PuTTY Download Page
2. In Duplicati select the "Duplicati Options" -> "SSH" and select the pdftp.exe as SFTP program.
3. Using the "Duplicati Assistant", edit your backup to use the external SFTP program.

It took me an hour to track this issue, so I created this "share your knowledge" question on Stack Overflow. Hope it helps someone.