I am having the same problem I have many dozens of sites on the same server running X and they verify fine. I tried to revoke and use a different license it didn't work either. Here is the error message:
cURL error 60: Peer certificate cannot be authenticated with known CA certificates
edit: I was able to get it to work by deleting the cornerstone plugin and the X folders (including child) via FTP and the install them the same way. I was then able to validate my license.