In plain English this happens after you upgrade VBOX and it rebuilds kernel modules based on a newer kernel than you are running (eg. you upgraded your kernel but haven't booted into it).
It is too bad there is not a standard work around that would allow it to realize this and keep old kernel modules and use them if it detects your current kernel is older than the most recently build modules.
RTR3InitEx failed with rc=-1912 (rc=-1912)
The solution is to run '/etc/init.d/vboxdrv setup' and sometimes happens when upgrading VBox and the kernel modules don't get upgraded with it. Sometimes the above doesn't work until you reboot because sometimes other unknown processes (even the file manager possibly) may be locking the old module from being unloaded.
Failed to open a session for the virtual machine XP.
The virtual machine 'XP' has terminated unexpectedly during startup with exit code 1.........