For those who still follow, pretty good new here ! I finally succeeded installing Virtualbox on HighSierra !
As I expected, some security rules were set without I could check/find them. The Security and Confidentiality panel does not give any details, only one app were sometimes shown as blocked by the system and even I allowed it, it failed (message remained in the GUI for the blocked application and the application could not be used better). Some apps were prevented from starting but I had no way to find which ones and thus to fix them...
By chance, I could progress when starting my mac in
Safe mode. Here, the Security and Confidentiality panel works much better. When trying to "Allow" the app that is is showed in the panel, this opens a list box with the list of currently blocked applications !! Here you can tick the one(s) you want to release and effect in the GUI is immediate. In my case, releasing "Oracle" triggered a restart and once back in normal mode, the Security and Confidentiality panel showed no more blocked application AND installation of Virtualbox succeeds !
Even better, Virtualbox works
! kernel extensions are properly loaded and running !
So, it's of course a set of High Sierra troubles and its a pity the information reported in safe mode is not the same as in normal mode. I requested that to Apple for days and no one could tell me the list of applications that were blocked... It's all here in safe mode.
I hope this also helps others who face the kernel driver loading error, since the root cause seems to be the same...