After upgrading to OS X Mavericks I was experiencing strange errors when trying to start a VirtualBox VM. e.g.
Progress state: NS_ERROR_FAILURE
VBoxManage: error: Failed to create the host-only adapter
VBoxManage: error: VBoxNetAdpCtl: Error while adding new interface: failed to open /dev/vboxnetctl: No such file or directory
VBoxManage: error: Details: code NS_ERROR_FAILURE (0x80004005), component HostNetworkInterface, interface IHostNetworkInterface
VBoxManage: error: Context: "int handleCreate(HandlerArg*, int, int*)" at line 66 of file VBoxManageHostonly.cpp
Problem was solved by running the following command in the command line:
sudo "/Library/Application Support/VirtualBox/LaunchDaemons/VirtualBoxStartup.sh" restart
It will install the VirtualBox binaries and kernel extensions again. The OS X will remove them at boot so you'll need to run it after every boot.