VirtualBox refuses to start VM: `The device helper structure version has changed`

This is typical behavior of VirtualBox after it has been upgraded by Ubuntu while it was running.

Solution:

• Stop VirtualBox
• Download the latest extension pack at https://www.virtualbox.org/wiki/Downloads (it is the same for all platforms)
• Install the extension
• Restart Ubuntu

It should now work with any VM that worked previously.

Actually, the log contains this very tip: If you have upgraded VirtualBox recently, please make sure you have terminated all VMs and upgraded any extension packs.

If you need to get this running and you don't have internet you can remove the extension pack:

FIle->Preferences->Extensions

And set USB controller to 1.1 in machine you want to boot. Select it, then:

Machine->Settings->USB->Set it to 1.1 (or disable it)

That should allow you to boot. You will not have extensions available, but is may get you through until you get internet. When you get online follow Nicolas Raoul' answer