Server's host key is invalid

How do I fix this error: "Server's host key is invalid".

I am using WinSCP 6.5.5 on a Windows 11 PC, connecting to a CoreFTP Server on a Windows 7 PC. I can connect to the same server using a CoreFTP client, but not WinSCP. I can connect to a different server on a Windows 11 PC using WinSCP.

I was expecting to be shown a warning message saying the server's host key is not cached, showing me the server's key fingerprint, and asking if I trusted it and wanted to continue, but all I get is "Server's host key is invalid".

Thanks.