Support OpenSSH 8.8 #4615 has this stack in Duplicati issues and links to a fixed SSH.NET issue.
Results of relevant openssl version and ssh -V might tell us if this version incompatibility exists.
If that is this problem, a new Duplicati would be needed to pick up the SSH.NET that has their fixes.
So a LinuxServer image, not a Duplicati. Chances are low that you’ll get one from the Duplicati team.
Occasionally the Duplicati developers will do private test builds, but they’re usually not Dockers at all.