The quick
Fixed. I had to import Portal's TLS cert.
Key problem
It was an encryption trust problem.
I triggered a validation of the "Registered Server Connections" by clicking the link "Data Stores" on the left and also hitting "Validate All". Then I clicked the Logs link and saw the error:
An SSL Handshake error has occurred when attempting to communicate with URL: "https://ourPortalServerUrl:7443/arcgis/sharing/community/self?f=json&token=encodedTokenTextHere.". Please make sure you have imported the SSL certificate from "https://ourPortalServerUrl:7443/arcgis/sharing/community/self?f=json&token=encodedTokenTextHere." into GeoEvent's site via the ArcGIS Server Admin API. Root Cause: "PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target".
Fix
1. Navigated to the GeoEvent server's ArcGIS Server admin page:
https://ourgeoeventurl/server/admin/machines/ourGeoEventUrl/sslcertificates
2. Clicked: "importExistingServerCertificate"
3. Uploaded our Portal server's .pfx file (I already had the root certificate imported).
4. Navigate to the GeoEvent "Data Stores" page. It now shows a green checkbox for the "Default" server connection.
-Pete