If you have recently upgraded to Proxmox 9, it looks like Veeam Backup & Replication does not quite as of yet support Proxmox 9 VE Server. While I could add the PVE9 host to Veeam and install the worker node, attempting to restore disks from a VMware environment to the PVE9 host resulted in: Failed to prepare disks for restore error.
I was restoring the disks to an LVM-Thin NVMe drive as RAW format. Below is an example of the error message I received for any of the VMs I attempted to restore.
Also, I wanted to see if I saw low-level errors on the Proxmox host, so I watched the journalctl command:
journalctl -f | grep -i qemu
I saw these errors appear once the disk restore got underway, something to the effect of Use of uninitialized value in multiplication (*) at...
I downgraded the host back to Proxmox 8.4 VE Server and was able to carry out the operation without issue. This is one of the blockers usually when new versions of hypervisors come out. Backup solutions usually take a while to release updates to cover the new version of the hypervisor. This looks to be the case with the current Veeam version and support for Proxmox 9.