Unable to open services published from ArcGIS Pro in Portal for ArcGIS Map Viewer

Description

When attempting to open services published from ArcGIS Pro in Portal for ArcGIS Map Viewer, services are not loaded, and the following errors are returned in Map Viewer:

Error: 
Unable to add layer <layer name>
Unable to load <item URL> status: 999

Error: 
Unable to add layer <layer name>
The layer, <layer name>, cannot be added to the map.

Cause

• Mismatch of the service URL between Portal for ArcGIS and the ArcGIS Server REST endpoint. This may happen when changes were made to the deployment in the past, where the Service URL or Server URL on the server was changed or updated, resulting in the mismatch.
• ArcGIS Web Adaptor is not configured.

Solution or Workaround

Depending on the cause, the workarounds provided below are applicable.

Update the serverUrl parameter in the ArcGIS Server Administrator Directory

Warning:
Ensure the following diagnostic findings are fulfilled before proceeding with the workflow:
• The Service URL in the ArcGIS Server Manager Sharing page is incorrect
• The serverUrl parameter in the ArcGIS Server Administrator Directory Portal properties textbox is incorrect
• The Server URL in the ArcGIS Portal Directory Server List page is correct

1. Log in to the ArcGIS Server Administrator Directory with an administrator account.
2. Click security > config > update.
3. In the Portal properties text box, for the serverUrl parameter, specify the correct URL.
4. Click Update.
5. Restart ArcGIS Server. Refer to How To: Start and stop ArcGIS Server machines using the ArcGIS Server Administrator Directory for instructions.
6. Overwrite or republish existing (non-hosted) services to apply the updated server URL. Refer to ArcGIS Pro: Overwrite a map service for more information.

Configure ArcGIS Web Adaptor

Configure the web adaptor to work with the server site. Refer to ArcGIS Enterprise: Configure ArcGIS Web Adaptor for more information.