Unable to load //<OrganizationName>.maps.arcgis.com/sharing/rest/portals/?f=json status: 0

Error Message

ArcGIS Online allows the sharing of items from ArcGIS Server within an organization for collaboration and management of data between organizational members. However, adding or accessing an item from ArcGIS Server via a GeoJSON URL fails, and returns the following error message:

Error:   
Unable to load //<OrganizationName>.maps.arcgis.com/sharing/rest/portals/?f=json status: 0

Additionally, adding or accessing an item in ArcGIS Online from Portal for ArcGIS fails, and returns the following error message:

Error:   
Unable to load https://<MachineName>.<Domain>.com/<WebAdaptor>/<RESTFeatureServiceDirectory> status: 0

Cause

• The domain of the item is not authorized in the organization's settings.
• The ArcGIS Online list of trusted servers contains the incorrect server URL, or contains the item's full URI.

Note:
In ArcGIS Online, navigate to Organization > Settings > Security, and scroll down to Trusted servers to check the list of trusted servers in the organization.

Solution or Workaround

Note:
If the list of trusted servers contains the item's full URI, remove the URI from the list before adding the correct server URL.

In ArcGIS Online, add the server URL of the item to the organization’s list of trusted servers to authorize the domain in the organization.

1. In ArcGIS Online, navigate to the organization's security settings. Refer to Steps 1 to 3 in ArcGIS Online: Configure security settings to do so.
2. Under Trusted servers, click Add trusted servers.
3. In the text dialog box, specify the server URL containing the domain, and click Add server. Refer to ArcGIS Online: Trusted servers for more information regarding the URL format.