This entry from Security Metrics indicates that it is possible to determine that your server is a server and not a printer, or a router or some other device.
Knowing that this device is a server does not indicate a vulnerability.
This is not a vulnerability. In most cases, ClearOS is clearly operating with public facing services which do indeed reveal that this isn't a printer, router or otherwise. Displaying as something that we are not or obfuscating that we are not a server is not important as a security risk.