(further off-topic) Duplicati’s manual is a totally separate GitHub project, often in catch-up mode.
S3 Compatible did catch up with 2.0.6.1, but leaves no info on underscored options for earlier…
Testing backup configuration and “clean” backups might have been thrown off by the mismatch.
(more on-topic) I wonder if Duplicati manual can permalink to the in-use version for tech detail?
https://github.com/sshnet/SSH.NET/tree/2020.0.1 says lots for those who really want to know it.
The format change is obscure even with that. One almost needs OpenSSH release note on it…
https://www.openssh.com/txt/release-7.8
Potentially-incompatible changes
================================
This release includes a number of changes that may affect existing
configurations:
* ssh-keygen(1): write OpenSSH format private keys by default
instead of using OpenSSL's PEM format. The OpenSSH format,
supported in OpenSSH releases since 2014 and described in the
PROTOCOL.key file in the source distribution, offers substantially
better protection against offline password guessing and supports
key comments in private keys. If necessary, it is possible to write
old PEM-style keys by adding "-m PEM" to ssh-keygen's arguments
when generating or updating a key.