I have installed the virtual box 5.2.12 on my Macbook (High Sierra)
Then I've installed Ubuntu thanks to ubuntu-18.04.desktop-amd64.iso.
So far so good even though it is really slow.
Then I've installed (sudo apt-get install) gcc, make, perl in order to install VirtualBox Extension Pack (Oracle_VM_VirtualBox_Extension_Pack-5.2.12.vbox-extpack).
Finally reboot the virtual machine and I'm stuck.
[OK] started daemon for power management
stopping User Manager for UID 120...
I found similar threads but I did not find the right way to fix it!