Unable to add resources to an ArcGIS Online item

Description

When adding a resource, for example an image, to an ArcGIS Online item such as an ArcGIS StoryMaps or ArcGIS Dashboards application, unexpected behavior such as continuous loading occurs. Examining the network traffic reveals the following error:

Response:
{"error":{"code":400,"messageCode":"CONT_00945","message":"Exceeded total number of files for this resource","details":[]}}

Cause

The number of resources added as part of the ArcGIS Online item exceeds the 1,000 limit. The total number of resources that are part of an item can be checked using the following ArcGIS REST API request:

https://www.arcgis.com/sharing/rest/content/items/ID-of-story/resources?f=json&token=user-token-value

Note:
To generate a token for the user token value, use this link: https://www.arcgis.com/sharing/generateToken?f=json&username=USERNAME&password=PASSWORD&referer=https://www.arcgis.com

Solution or Workaround

Delete unnecessary resources associated with the item, or recreate the item. Alternatively, add the resource using a URL. Refer to ArcGIS StoryMaps: Add media for more information.