I tried that too. I have the plink.exe command that starts a remote
rdiff-backup on the linux-server and it works from the commandline on the
windows-server.
However, rdiff-backup is unable to use plink.exe and fails with the same
error.
Googling the error shows that it has been seen with connection from Linux
clients, so I don't think it relates to Windows. Are you confident that you
have the same versions of rdiff-backup on client and on server? Otherwise I
am out of ideas (I don't have Windows 2019 Server).