Wayland desktop on Ubuntu 17.10 after install/upgrade a VirtualBox Extension Pack may get a “can’t find Xauthority” popup error. The fix is to revert to X11 as recommended by Ubuntu.
- on login screen select X11 login
- install the VirtualBox Expansion Pack
- if you desire, logout and switch back to the default login (with Wayland). VirtualBox will work in Wayland.
Ubuntu 18.04 defaults to X11 once again.