When adding a new VM (in this example the nextcloud appliance VM from https://www.hanssonit.se/nextcloud-vm/) to an old version of proxmox like version 6 (debian 10), the VM might not boot and stay stuck showing Booting from Hard Disk ...
Solution
- Update your proxmox system, e.g. from version 6 (debian 10 - "buster") to promox version 7 (debian 11 - "bullseye"), see https://pve.proxmox.com/wiki/Upgrade_from_6.x_to_7.0
- Import the VM again and start it
