I have the same problem. The pve-manager downgrade did not help me, but helped downgrade:
- pve-manager = 6.3-3
- pve-qemu-kvm = 5.1.0-8
- libproxmox-backup-qemu0 = 1.0.2-1
and reboot
Additional information, after proxmox upgrading(pve-qemu-kvm), VM with windows...