A generic error message is returned when configuring ArcGIS Web Adaptor for ArcGIS Server

Description

Configuring ArcGIS Web Adaptor for ArcGIS Server fails and returns the following generic error:

Error:   
Bad syntax in request. (status code 400)

Cause

The fully qualified domain name (FQDN) of the machine contains the HTTPS protocol and the port number. As of ArcGIS Web Adaptor 10.8, the FQDN of the machine does not require the HTTPS protocol and the port number as part of its URL format.

Solution or Workaround

Use a URL format without the HTTPS protocol and port number when adding the portal URL during the ArcGIS Web Adaptor configuration process. The following is a sample of a fully qualified domain name URL:

<portal>.<domain>.com