When choosing 'Add to ...' from an Incident on Endpoint Detection and Response (SEDR) Appliance, the list never generates or continually spins.
The Appliance no longer has any entity data for the endpoints or files referenced in the Incident.
This can occur if an Endpoint is unenrolled from the SEDR Appliance and all references related to the client or clients has been purged. Make sure to close all previous Incidents for those clients before proceeding to new Incidents.