I am using version 5.0.7450.0 of your product within visual studio, .NET framework 4.8. I am getting the CryptographicException which reports: "Invalid Private Key Format". There is no password for this file, the code is:
var privateKey = new SshPrivateKey(@"c:\downloads\mypk.ppk");
I have been able to connect via FileZilla with the same key file. It's in the format:
where I've commented out the main info with xxxx.