Import the vmdk disk file with the correct format (vmdk) : qm importdisk 101 Nextcloud-AIO-disk001.vmdk volume1 -format vmdk
Change the BIOS setting to "OVMF (UEFI)" (it seems the official image is configured with UEFI, that's what I saw when creating the VM on Virtualbox)
Create a UEFI Disk (Hardware section of the VM)
Set the boot disk as a SATA disk (it was configured like this when running on Virtualbox)
Do not forget to add the new disk in the Boot Order (Options section of the VM).
login: ncadmin
pass: nextcloud