There is an issue in the Melbourne Availability zone affecting Snapshots of instances launched before 04/02/2019.
Snapshotting may fail and will cause the running instance to stop, requiring a restart.
This affects instances in the melbourne-qh2, melbourne-qh2-uom and melbourne-np cells.
Hard-rebooting the instance before snapshotting will alleviate the problem.
The UoM cloud team are currently investigating a solution and will post further updates here.