Sorry for the delayed response,
There is a dedicated storage for virtual disks (so not stored on the failed system disk)? - Yes, VMs are not stored on failed system disk.
Did you create a backup of your "/etc/pve/qemu-server" folder on your OS disks root filesystem? - No
Or is that PVE server part of a cluster? - Yes
Or do you have some VZDump/PBS backups of those VMs? - LZO backup available in NFS storage, however i dont think its of use, b'coz of old backup copies.