Unable to establish connection to Portal URL

Error Message

Attempting to connect to Portal for ArcGIS from ArcGIS Pro returns the error:

Error:   
Unable to establish connection to Portal URL.

This happens when setting Portal for ArcGIS as an Active Portal. When attempting to sign in to Portal for ArcGIS, ArcGIS Pro sends two requests: text/XML and text/HTML. The connection fails if either one of these requests is filtered. For additional information on Request Filtering, consult Configure Request Filtering in IIS.

Cause

Request filters are set on the web adaptor via Internet Information Services (IIS) Manager, preventing connections from ArcGIS Pro. The filtered requests are XML and HTML.

Solution or Workaround

Disable the relevant request filters on the web adaptors via IIS Manager. This is done by the user's IT department.