Unable to access the ArcGIS Web AppBuilder page after specifying a portal URL

Description

Attempts to access ArcGIS Web AppBuilder after specifying the organization's portal URL fail, with the following error message returned:

Error:
Unable to access https://<webadaptorhost>.<domain>.com/<webadaptorname>/home 
A server with the specified hostname could not be found.

Cause

• The Secure Sockets Layer (SSL) certificate used in Portal for ArcGIS is expired or untrusted.
• The SSL certificate used in Portal for ArcGIS does not contain a Subject Alternative Name (SAN).
• ArcGIS Web AppBuilder is used in an unsupported browser. Refer to ArcGIS Web AppBuilder: Supported browsers for more information.
• The portal URL is configured to have restricted cross-domain requests in the Portal for ArcGIS security settings.

Solution or Workaround

Import a new trustedSSL certificate containing a Subject Alternate Name (SAN) in Portal for ArcGIS

Import a trusted certificate with a Subject Alternative Name (SAN) issued to the fully-qualified domain name of the machine where ArcGIS Web Adaptor is installed. Refer to Portal for ArcGIS: Configure Portal for ArcGIS to use the CA-signed certificate for steps on importing a new SSL certificate in Portal for ArcGIS.

Remove the portal URL from the cross-domain requests list in the Portal for ArcGIS security settings

1. Log in to Portal for ArcGIS as an administrator.
2. Navigate to Organization > Settings > Security.
3. Under Allow Origins, remove the portal URL entry.
4. Browse to the following directory in the ArcGIS Web AppBuilder folder:

arcgis-web-appbuilder-2.x\ArcGISWebAppBuilder\server

5. Unregister any previous portal connection instances by deleting the signininfo.json file.
6. Refresh the browser running ArcGIS Web AppBuilder.
7. Specify the portal URL, and click Continue.
8. Enter the app ID, and click Continue.