It does look like it’s an error with reading the format. Did Putty make a .ppk file as private key?
And does your file look correct when you open it in a text editor?
It should looke something like this:
-----BEGIN RSA PRIVATE KEY-----
...
-----END RSA PRIVATE KEY-----
but .ppk’s usually look like this:
PuTTY-User-Key-File-2: ssh-rsa
Encryption: none
Comment: imported-openssh-key
Public-Lines: 6
...
Private-Lines: 14
...
It should also be noted that Renci.SshNet does not support anything but RSA and DSA (Non-RSA SSH key rejected with poor error message · Issue #2950 · duplicati/duplicati · GitHub)