Cannot add a web map service (WMS) to the web map when accessing Portal for ArcGIS through HTTPS.

Additional Information

Workaround

1. Log into Portal for ArcGIS.
2. Click Organization and Edit settings.
3. Click Security and make sure that 'Allow access to the portal through HTTPS only' is NOT enabled.
4. Click Save.
5. Click Map.
6. Change the URL at the top of the page to http://portal.domain.com/webadaptor if it is not already.
7. Click Add and 'Add layer from web'.
8. Change type to 'A WMS OGC Web Service'.
9. Enter the URL: http://sgx.geodatenzentrum.de/wms_topplus_open?SERVICE=WMS&REQUEST=GetCapabilities&VERSION=1.3.0 or https://sgx.geodatenzentrum.de/wms_topplus_open?SERVICE=WMS&REQUEST=GetCapabilities&VERSION=1.3.0
10. Click 'Add Layer'.

Steps to Reproduce