If your KMS is running on a Windows Server 2012R2 and you want to activate Windows Server 2019, you might get error messages.
If your KMS server is running on Windows Server 2012R2 and you want to install Windows 2019, you might get errors when installing your key into the KMS. Also your server might have trouble activating itself on the KMS. Error messages are indicating that the KMS is not available (Error code: 0xC004F074).
To resolve the issue, you need to install the december updates on the KMS server (See the preview update for the change log or the final update containing the fix.