SessionOptions.GiveUpSecurityAndAcceptAnyTlsHostCertificate = true has no effect

Even when I set this option, I still get the exception when using the .NET library:

ArgumentException: SessionOptions.Protocol is Protocol.Sftp or Protocol.Scp, but SessionOptions.HostKey is not set.

I am using SCP with an SSH private key file path specified, if that's relevant. As far as I understand it, setting SessionOptions.GiveUpSecurityAndAcceptAnyTlsHostCertificate should obviate the need to specify a host key, as stated in the documentation and forums.

No, it's GiveUpSecurityAndAcceptAnySshHostKey:
https://winscp.net/eng/docs/library_sessionoptions

Make sure you understand what you are doing when using the property!