This problem (still or again) occurs with ubu 18.04 (host), 18.10 (guest) and virtualbox 5.2.18.
After installing the guest additions at the host, the guest system will boot, but x server will crash and state
"Warning, couldn't open module vboxvideo".
Modprobe vboxvideo will load the module, nevertheless X server will crash with the same message.
This problem (still or again) occurs with ubu 18.04 (host), 18.10 (guest) and virtualbox 5.2.18.
After installing the guest additions at the host, the guest system will boot, but x server will crash and state
"Warning, couldn't open module vboxvideo".
Modprobe vboxvideo will load the module, nevertheless X server will crash with the same message.