Ubuntu guest stuck in startup loop

[kenatsun]

I suddenly can't start up my Ubuntu guest, which had been working fine for several weeks. The boot sequence goes fine until the desktop appears and prompts me for my password. (See attached screen shot.) I key in the pw and hit enter. After a few seconds, the guest screen briefly goes blank. Then the password prompt appears again, and so on forever. No error message appears, just this infinite loop.

I know I'm entering the right password because when I enter a different (invalid) password, I get an explicit "invalid password, please try again" error message.

The screen also offers a "guest" login. Trying to sign in as guest leads to the same loop, except without asking me for a password.

The one difference between when it used to work and now when it doesn't work is: it worked when I was back home in Michigan, and now I have taken the machine to a conference in California, and it doesn't work. I don't see why the change of location should make any difference in starting up a VM. Apart from the problem reported in this thread, the computer works just the same in both locations.

~ Thanks, Ken

Details:
VBox version: 5.1.2 r108956
Guest: Ubuntu 16.04 LTS
Host: Windows 7 Pro
Hardware: Lenovo T410 laptop w/ 8 GB RAM

[kenatsun]

Well, it's working again, and all I did that apparently fixed it was to return home to Michigan!

I have no idea if the change of location what fixed it, and if that's what fixed it I have no idea why. But there it is.

~ Ken