My personal notes about solutions for VirtualBox issues I faced.

Issues

Black screen after installing Guest Additions

Try to disable 3D Acceleration in the VM Settings.

VirtualBox shared folder permission

You can not read the content of the shared folder inside Ubuntu VM.

Solution: Run

sudo adduser $USER vboxsf

from VM terminal, then reboot or logout and login.

Increase the video memory to 256MB (maximum value)

Solution: Run

VBoxManage modifyvm "{virtual-machine-name}" --vram 256

from host terminal.