[ubuntu] VM 18.04 beta2 fail when I install guest additions under 16.04

When I install Ubuntu 18.04 beta2, it works fine -- until I reboot after installing the guest additions.
The boot behavior seems normal at first, but then it cycles rapidly between littlewindow and fullscreenwindow a couple of times; then it halts without ever arriving at a desktop display. I've attached the last screenful of boot messages.

Attached Images

• beaver_defect.png (38.5 KB)