Recovering Saved Machine State in Virtual Box

I went to reboot my machine, but Virtual Box popped up and asked me what I wanted to do with the currently running VM. It gave me the following options:

• Save the machine state
• Send the shutdown signal
• Power off the machine
• Restore current snapshot 'Init 4'

I wanted to resume the VM exactly where I left off after the reboot, so I picked the first option. After a few moments of a progress bar where it appeared to be saving the machine's state, Virtual Box finished and then quit.

A bit later when I restarted Virtual Box I was informed of a minor upgrade (5.22 to 5.24). So I quit and installed the update. But after this when I tried to restore my VM it appeared to be booting it from scratch, rather than restoring, and I got the error:

FATAL: Could not read from the boot medum! System halted.

After some digging I discovered that there was no storage attached to the IDE controller, so I attached the VM image. After this the VM booted, but the image appears to be several months old, despite the fact that the snapshot pane appears to show that I am on the newest snapshot.

When I look in the Snapshots folder for my VM I see three files dated today, two around the time the original save was done (a .vmdk file and a .sav file) and another vmdk file around the time I rebooted. So it really looks like the save is there, but how do I actually use it?

Update: I tried using the VBoxManage adoptstate command on the .sav file, but now when I try to start up it fails with this error:

VBoxManage: error: pci#0: New device in slot 0x28, ichac97 (vendor=0x8086 device=0x2415) [ver=4 pass=final] (VERR_SSM_LOAD_CONFIG_MISMATCH)
VBoxManage: error: Details: code NS_ERROR_FAILURE (0x80004005), component ConsoleWrap, interface IConsole