Unable to delete a device—it's greyed out, but still visible in XMS web panel.
Resolution
Stop the XoperoONEManagementService service and remove the DeviceModule directory from the XMS installation folder:
For Windows:C:\ProgramData\Xopero ONE\Xopero ONE Backup&Recovery Service\
For Linux:/opt/XoperoONEMangementService/
Executing this operation will remove all devices from your Xopero ONE instance. You will need to re-add each device and reassign them to all backup plans.