When opening a VM in VirtualBox I encountered the error message:
The device helper structure version has changed.
If you have upgraded VirtualBox recently, please make sure you have terminated all VMs and upgraded any extension packs. If this error persists, try re-installing VirtualBox. (VERR_PDM_DEVHLPR3_VERSION_MISMATCH).
Result Code: NS_ERROR_FAILURE (0x80004005)
There's lots of blogs and the answer is hidden away in VBox forums, but:
I had installed VirtualBox from the Ubuntu Software Centre. I should have downloaded it from the virtualbox website.
So, I uninstalled from Software Centre, downloaded, reinstalled, and Voila.. all good