I start the machine, login, start VirtualBox and get
Invalid MIT-MAGIC-COOKIE-1 keyFailed to open the X11 display!
I then click the PC button, select leave, Switch User, New User, and get a second user session.
I login as a second user (same username), start VirtualBox and it work.
When I switch between the two sessions, the first one (that does not work) is ctrl-alt-F7 and the second one (that does work) is ctrl-alt-F8.
This is a new problem that started after an update a few weeks ago.