Unable to add new deploy target (SSH)

I am trying to add a new deployment target (it’s a Linux host - AWS EC2).

I’ve added an account in Infrastructure, and added the private key of the instance. This is the same private key file I use locally to connect.

Note: I am able to connect via ssh or scp from other hosts, using the private key file. I have verified that I am able to connect (via ssh) from the server running Octopus, so I know it’s not a routing issue.

BUT - when I attempt to add this instance as a deployment target in Octopus, and then verify connectivity, Octopus consistently fails to connect.

If I enter the full details manually (including the fingerprint) I also get an error…

The only thing I am not 100% sure of is the fingerprint I added. I generally see a different error in SSH connections if the fingerprint is wrong or has changed.