Cannot start any Virtual Machine anymore on High Sierra

I just switched from Sierra to High Sierra for some reason, and Virtualbox doesn't work anymore. All my machines (Debian, Windows etc) show that message when started:

The virtual machine 'Serveur PHP 5' has terminated unexpectedly during startup with exit code 1 (0x1).

Error code: 
NS_ERROR_FAILURE (0x80004005)

Component: 
MachineWrap

Interface: 
IMachine {5047460a-265d-4538-b23e-ddba5fb84976}

Any idea?

Many thanks