The disk status is critical (vendor status: 2-Unhealthy)!
The health of the mirror could be impacted by the loss of a single disk so another alert might have occurred for the disk itself.
The status is reported in the component's 'phys_disk_status' property. The Operational Status, shown in property 'phys_disk_oper_status' might provide more information about the problem.
Review the node's events in log 'Application and Service Logs\Microsoft\Windows\StorageSpaces-Driver\Operational' for further details.
To resolve this issue, contact Microsoft support and provide them with the alert name and details. Microsoft Support will help you to understand the failure and help you to bring the node back online or into a healthy state.