Summary

When invoking REST API: /rest/agile/1.0/board/<board-id>/backlog no issues are returned an an error message: The backlog is not available on Kanban boards is returned. Unlike when invoking the API with a board ID from Scrum project you, then issues are returned.

Steps to replicate

1. Spawn Jira Server 7.5.0 with software installed
2. Create a Kanban Project with Kanplan enabled (via configure board -> columns -> move status to Kanban Backlog)
3. Create issues in Backlog via "Backlog" icon in sidebar
4. Invoke REST API endpoint: /rest/agile/1.0/board/<board-id>/backlog and you'd get a response:

   {
   "errorMessages": [
   "The backlog is not available on Kanban boards"
   ],
   "errors": {}
   }
   

Note:
Endpoint working for Cloud