After Ubuntu kernel upgrades (which may happen unnoticed by the user, depending on your upgrade settings), VirtualBox 5.0 machines may fail to start.
The error message tries to be helpful and suggest to run the following command:
$ sudo /sbin/rcvboxdrv setup
However, this command fails:
Bad argument setup
sudo /usr/lib/virtualbox/vboxdrv.sh setup
The most permanent solution however is to upgrade to VirtualBox 5.0.16 (or newer), where the bug has been fixed. This version was released on March 4.