Though the DigiCert Wildcard Certificate is listed here for FileMaker Server 15:
After installation (and FM Server restart) you still get the warning on the Database Server > Security tab:
"Warning: The custom SSL certificate installed on this server does not originate from a Certificate Authority (CA) supported by FileMaker. FileMaker Go cannot be used to connect securely with this certificate."
It turns out, however, that FileMaker Go 15 and FileMaker Pro 15 can connect without issue and show it as a valid certificate with a green lock. So, the warning in the Admin Console seems to be a bug. Attached I've included a screenshot of the certificate details and warning.
But, this certificate (or, at least this certificate and not the default certificate) creates another issue. When I install this certificate on a Virtual Machine (Windows 2008 R2 SP1 on Citrix XenServer 6.5) that VM operates just fine. I can power it on and off and FileMaker Server 15 always seems to come back up. However, if I turn off the VM, clone the VM, and then turn the clone on (no changes to IP or MAC address or any other hardware settings) the FileMaker Server service refuses to start. In the Windows Application Event Logs the error "FileMaker Server Helper quitting! Network initialization error. (CORBA::INITIALIZE)" is found. If I remove serverCustom.pem from the CStore folder then FileMaker Server will start again on the cloned VM, but of course using only default SSL certificate.
Cloning VMs allows for rapid server deployments and is a means of server backups. Please investigate and correct.