Thanks jarugut, at first it didn't work again so i thought i had to have missed something... And I did... So just for people who might have the same issue as I did :
Make sure you create the folder "VirtualBoxVMs" before trying to install Virtualbox
Make sure your web station has a the right settings (nginx and not apache, soap setting activated), otherwise you'll have a "php error" when trying to run virtualbox after installing it.
Thanks again jarugut ;).