Issue
ISE Posture updates fail with the following error.

Solution
The issue is resolved by:
- Ensuring that the posture update URL is valid
- Ensure that proxy configuration is correct (if used)
- Ensuring that ISE trusts the certificate presented by the URL
- Navigate to the URL and make a copy of the certificate
- Navigate to Administration > System > Certificates > Trusted Certificates and add the downloaded certificate to the Trusted Certificate store
- Once uploaded, return back to ISE Posture Updates and update. You should no longer be presented with the above error
