Imported Virtualbox VMs fail to boot

[apparatus]

Is your VM installed in UEFI mode or in Legacy BIOS mode?
The qubes are booting in Legacy BIOS mode by default.
If your VM used UEFI then you can either convert it to Legacy BIOS mode before importing it as a qube or boot the qube in UEFI mode:

Importing Vmware VM (*.vmdk) into QubesOS

You can try to enable UEFI for your qube by setting uefi feature for it: qvm-features <vm-name> uefi 1 And add ovmf.bin in your dom0: