The KopiaUI (Windows version) setup to a remote Linux SFTP server asks for the path to the “known_hosts” file (or its contents).
This file is typically available in a local Linux installation - but not in Windows. For example, my putty application stores this in the Windows registry - which is not accessible via path. And the contents of the registry key were unable to be parsed by KopiaUI.
I believe this is a Feature Request, unless there is a work-around that I haven’t discovered yet.