In your case, first install Ubuntu when it asks you to restart. Pause the VM from Virtual Box and then make a copy of complete folder where you have stored your VM. You can find this path after right clicking on the VM and then going into settings and HHD location.
Now restore the VM State (Just Start it). Then, click on restart once it shut's down. Pause the VM again and them remove the Installation media / ISO what so ever it is. then restore / start the VM your VM will boot properly. In case it does not let me know will let you know what else can be done in this case.
@pinouchon You need to turn off your VM then go into the settings of that VM by right clicking it. Then, go to display as shown in the below Screenshot and check accelerate 3D graphics and then click 'OK'. and start your VM it will start working fine. Let me know.
These small tricks should work. However, there are more complicated solutions to these issues which we can try if this does not work.