After enabling 'HTTP Only' from the ArcGIS Server Administrator Directory, attempting to access any ArcGIS Enterprise component via the ArcGIS Web Adaptor URL fails, and returns the following error:
Error: The ArcGIS Web Adaptor has been configured with SSL/HTTPS. Please enable SSL/HTTPS for your ArcGIS Server Site.
Attempting to access any ArcGIS Enterprise component via Internet Information Services (IIS) Manager also fails.
The WebContextURL property is set in Portal for ArcGIS, and the ArcGIS Enterprise components can only be accessed via the DNS alias URL.
Access the ArcGIS Server Administrator Directory via the DNS alias set as the WebContextURL, and change the Protocol to HTTP and HTTPS.
The ArcGIS Enterprise components can now be accessed via the ArcGIS Web Adaptor URLs.