Unable to validate TLS certificate on ArcGIS Server administration URL

Error Message

When attempting to validate a hosting server, the following error is encountered:

Unable to validate TLS certificate on ArcGIS Server administration URL. Ensure the root certificate is trusted or the certificate common name matches the URL hostname.

Solution or Workaround

Import ArcGIS Server's root/intermediate SSL certificates into Portal's key store. See the article: Configuring the portal to trust certificates from your certifying authority