If you setting up VirtualBox on a server you will probably try to set users and groups by you preferences. Changing UIDs and GUIs by your preference after VirtualBox is installed could bring you in a inconsistent situation and you get this message:
VBoxManage: error: Failed to create the VirtualBox object!
VBoxManage: error: Code NS_ERROR_FACTORY_NOT_REGISTERED (0x80040154) - Class not registered (extended info not available)
VBoxManage: error: Most likely, the VirtualBox COM server is not running or failed to start.
This cryptic message is in fact saying you that something is wrong with users or groups. But what? Not the libraries, not any file created during the VirtualBox installation but… the files created in the /tmp folder. Simply delete all related thing, even the hidden ones, and you will be fine again!