This could also happen if there was another SSH server earlier on that hostname or IP address.
Go to the web interface and open the SSH application in System section. It will show you the fingerprint of the SSH server running on your FreedomBox. If this fingerprint matches the one you are seeing in the message you have posted (the part after SHA256), then all is good. You can edit the file C:\Users\markw/.ssh/known_hosts on your windows machine and delete the line number 2 and problem will be solved. Do not do this if the fingerprint does not match. In that case, double check that you are connecting to the correct server (IP address or hostname).