Unable to load the ArcGIS Online basemap gallery

Description

When clicking the Basemap button in the ArcGIS Online map viewer, the full Esri basemap gallery fails to display.

Cause

Possible causes for this issue are:

• The configured security settings allows for mixed content of HTTP and HTTPS. The common error displayed when faced with this issue is "403 Forbidden" error. This is an HTTP status code that indicates access to the particular web page is denied.

Note:
The following ArcGIS Online Help document describes the behavior of using mixed content environment: ArcGIS Online Help: Troubleshoot.

• Browser cache issues
• The default Basemap Gallery configured for the organization has changed

Solution or Workaround

The following are possible solutions for this issue:

• Use the ArcGIS Online Health Dashboard
  The ArcGIS Online Health Dashboard is a status page that provides current information regarding the service availability of ArcGIS Online. The information provided is for the overall health of the ArcGIS Online system. A Service disruption message is returned when a particular service becomes unavailable. If there is any known issue with the ArcGIS Online default basemaps it is listed here.  The ArcGIS Online Health Dashboard page can be viewed at: ArcGIS Online Health Dashboard.

• Configure ArcGIS Online with the appropriate security settings
  Certain layers do not display properly with mixed content. This also depends on the web browser used to view the content. The following ArcGIS Online help document describes the security options available and how to configure the security settings: Configure Security settings.

• Try another browser
  If the basemaps displays successfully in another browser, clearing the browser cache of the original browser may resolve the issue. 

• Check which basemap gallery has been configured for the organization
  Certain layers do not display properly with mixed content.

1. Log in to ArcGIS Online with an administrator account.
2. Click My Organization > Edit Settings > Map.
3. Take note of the group selected under Basemap Gallery.

The basemap gallery only displays the default basemaps if either Esri Default is selected, or if a custom group has been selected and the Esri Basemaps have been shared to that custom group.

• Re-associate the default Esri basemaps with the basemap gallery

1. Click Groups > CREATE A GROUP.
2. Give the newly-created group a name and leave all other options to default.
3. Click Save.

4. Click My Organization, click EDIT SETTINGS, and click Map.
5. From the Basemap Gallery option, click the drop-down arrow, and select the newly created group.
6. Check the option to Share the Esri default basemaps to this group when you click Save. Checking this option includes the default Esri basemaps within the newly created group. 

7. Click Save.
8. Click Map, and click the Basemap widget to verify if the basemap gallery is available.