Sometimes your filesystem gets stuck. No operations can be done and a “reboot” will just cause an indefinite waiting time for I/O. There’s a “magical” code that triggers a specific kernel condition - and a reboot.
Proxmox Backup Server (PBS) has been released. It's still in beta but is already perfectly usable. After many years, it's now possible to perform incremental backups of the VMs and thanks to the qemu dirty bitmaps, backups are also fast.