Issues building VirtualBox packer image of Windows Server 2012r2

Issue

I'm following this great how-to and all goes well until packer needs to shutdown the newly provisioned which fails with Cannot unregister the machine 'packer-virtualbox-iso-1469510168' while it is locked. See full log below at the end of this question.

I've done the following

• Check no stale VirtualBox VMs are left around with that name (all clear)
• Update VirtualBox to latest available (5.1.2 r108956)
• Reboot my host computer

but this consistently happens.

Environment

• Host OS - OS X El Capitan 10.11.5
• VirtualBox - 5.1.2 r108956
• Packer - 0.10.1
• Boxstarter - Current Master (last commit eb1cd9f86a2a8b1ee54cc14cc2c23ace93e01b96)

Log

2016/07/26 15:33:26 ui: ==> virtualbox-iso: Unregistering and deleting virtual machine...
==> virtualbox-iso: Unregistering and deleting virtual machine...
2016/07/26 15:33:26 packer: 2016/07/26 15:33:26 Executing VBoxManage: []string{"unregistervm", "packer-virtualbox-iso-1469510168", "--delete"}
2016/07/26 15:33:26 packer: 2016/07/26 15:33:26 stdout:
2016/07/26 15:33:26 packer: 2016/07/26 15:33:26 stderr: VBoxManage: error: Cannot unregister the machine 'packer-virtualbox-iso-1469510168' while it is locked
2016/07/26 15:33:26 packer: VBoxManage: error: Details: code VBOX_E_INVALID_OBJECT_STATE (0x80bb0007), component MachineWrap, interface IMachine, callee nsISupports
2016/07/26 15:33:26 packer: VBoxManage: error: Context: "Unregister(CleanupMode_DetachAllRetuHardDisksOnly, ComSafeArrayAsOutParam(aMedia))" at line 155 of file VBoxManageMisc.cpp
2016/07/26 15:33:26 packer: 2016/07/26 15:33:26 Executing VBoxManage: []string{"unregistervm", "packer-virtualbox-iso-1469510168", "--delete"}
2016/07/26 15:33:26 packer: 2016/07/26 15:33:26 stdout:
2016/07/26 15:33:26 packer: 2016/07/26 15:33:26 stderr: VBoxManage: error: Cannot unregister the machine 'packer-virtualbox-iso-1469510168' while it is locked
2016/07/26 15:33:26 packer: VBoxManage: error: Details: code VBOX_E_INVALID_OBJECT_STATE (0x80bb0007), component MachineWrap, interface IMachine, callee nsISupports
2016/07/26 15:33:26 packer: VBoxManage: error: Context: "Unregister(CleanupMode_DetachAllRetuHardDisksOnly, ComSafeArrayAsOutParam(aMedia))" at line 155 of file VBoxManageMisc.cpp
2016/07/26 15:33:27 packer: 2016/07/26 15:33:27 Executing VBoxManage: []string{"unregistervm", "packer-virtualbox-iso-1469510168", "--delete"}
2016/07/26 15:33:28 packer: 2016/07/26 15:33:28 stdout:
2016/07/26 15:33:28 packer: 2016/07/26 15:33:28 stderr: 0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%
2016/07/26 15:33:28 packer: 2016/07/26 15:33:28 Deleting floppy disk: /var/folders/d2/_yplf93n6qx_0h891qdv7dqsm9bttd/T/packer492700055

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