Failed to open a session for the virtual machine Ubuntu.
Unsupported version 17 of data unit 'cpum' (instance #1, pass 0xffffffff) (VERR_SSM_UNSUPPORTED_DATA_UNIT_VERSION).
Result Code: E_FAIL (0x80004005)
Component: Console
Interface: IConsole {8ab7c520-2442-4b66-8d74-4ff1e195d2b6}
I got this error code when I tried to open my virtual box this morning, which works perfectly fine yesterday. I saved my machine state before turning off the computer. I uninstalled Bluestacks this morning, and install(then unistall) Droid4X, which I think to be the source of problem, since Droid4X somehow was listed on my VirtualBox. Can someone explain to me why is this happening?
For me, this problem occurred while updating virtual box to the higher version and my virtual machine are in a saved state instance. So I have to Discard the Saved state. Then stared it as normal. It's works fine.

[SOLVED] To fix it, edit your vbox file (ex: MyWin7.vbox) And just remove the section "stateFile" in global configuration.
Before:
Machine uuid="{051685d4-f6b6-4994-a3cb-cc30271e280b}" name="MyWin7" OSType="WindowsXP_64" stateFile="Snapshots/2016-10-26T08-14-10-696037500Z.sav" snapshotFolder="Snapshots" lastStateChange="2016-11-07T11:07:35Z"
After:
Machine uuid="{051685d4-f6b6-4994-a3cb-cc30271e280b}" name="MyWin7" OSType="WindowsXP_64" snapshotFolder="Snapshots" lastStateChange="2016-11-07T11:07:35Z"
restart your VM
I recently ran into this issue with an error message:
"Unsupported version 4 of data unit 'pci'"
with a Ubuntu 14 VM on VirtualBox 5.1 for Mac. I originally used VB 5.2 when I setup the VM, saved it, then had to downgrade VBox (to use Vagrant), which I suppose caused this issue. The fix was similar, which is hard-rebooting the VM to skip the saved state. I did it from the UI though - right click, select Start/Normal Start. There would be a warning about discarding the saved sate but this wasn't a problem for me.
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