In versions 3.9 and higher this message will come up when you log into Spire Desktop using the server name or IP address. This is by design and you can just click Ignore.
However, this message should not come up when you are logging into your Hostname.
The Hostname looks something like cooperative-river-3842.spirelan.com
If this message comes up when you are logging into your Hostname, please do the below steps.
1. Rename C:\ProgramData\Spire\Server to C:\ProgramData\Spire\ServerBU on the server.
2. Run the Spire Server Installer again.
3. Select the same Hostname that you have been using, when asked, and a new certificate will be generated.
4. If everything works, then you can delete C:\ProgramData\Spire\ServerBU.