Thanks for the info. Here's the guru meditation:
00:00:12.581176 VMMDev: Guest Log: BIOS: Booting from Hard Disk...
00:00:12.588543 Display::i_handleDisplayResize: uScreenId=0 pvVRAM=0000000000000000 w=720 h=400 bpp=0 cbLine=0x0 flags=0x0 origin=0,0
00:00:12.935613 GUI: UIMediumEnumerator: Medium-enumeration finished!
00:00:15.865302 Changing the VM state from 'RUNNING' to 'GURU_MEDITATION'
00:00:15.865302 Console: Machine state changed to 'Stuck'
00:00:15.866465 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
00:00:15.866466 !!
00:00:15.866467 !! VCPU0: Guru Meditation 1155 (VINF_EM_TRIPLE_FAULT)
A triple fault is when something happened that was so screwed up that Virtualbox threw up its hands and said "Here, you handle this, I'm going home."
The VM "hardware started reading from the VM's hard disk, and:
00:05:08.489259 /IEM/CPU0/cInstructions 8835723 count
00:05:08.490020 /Public/Storage/AHCI0/Port0/BytesRead 1024 bytes
did a lot of crunching a great deal on 1k bytes from the disk, if I read correctly.
Looks like something in the VM got really bungeed. If the VM is important, try booting it with the Debian installer ISO and try a repair install. Otherwise try a new VM and see what happens.
FWIW Hyper-V is enabled on the host OS. However, that's not getting in the way as much as it used to, and Hyper-V is very integrated into the host OS now. You could try turning it off, but you'd be disabling a lot of supposed security structures. It's up to you whether you need them.
HMR3Init: Attempting fall back to NEM (Hyper-V is active)