SftpDrive complains that my serial number isn't valid

I have been trying to recover my serial number via http://www.expandrive.com/support. SftpDrive won't accept the key that's sent to me--it complains that it's invalid. I've sent email to support@sftpdrive.com--as the error message tells me to--twice, and haven't gotten a response. How can I get a working license key?