[Solved] unable to unlock screen after inactivity in Ubuntu Desktop 18.04

[ordinarysoul]

I am running VirtualBox 5.2.12r on Windows 10 host, and Ubuntu Desktop 18.04 as guest OS. Ubuntu locks after a few minutes of inactivity, and I am unable to unlock it. The locked screen does not respond to mouse clicks or keyboard inputs. I also tried "Input"->"Keyboard"-> "Insert Crtl+Alt+Del". The screen simply stays locked with time display. Any ideas? Many thanks.

Attached is the virtual box log.

[socratis]

Disable the 3D acceleration, see if that fixes it. If not, we're going to need a complete VBox.log, from a complete VM run:

• Gather the necessary information:
  1. Start the VM from cold-boot (not from a paused or saved state).
  2. Observe the error, i.e. let the screen saver kick in.
  3. Shutdown the VM (force close it if you have to).
• With the VM completely shut down (not paused or saved), right-click on the VM in the VirtualBox Manager and select "Show Log".
• Save only the first "VBox.log", ZIP it and attach it to your response. See the "Upload attachment" tab below the reply form.

BTW, I always disable any power optimizations and screen savers on my guests. There's absolutely no point...

[ordinarysoul]

3D acceleration is not installed. Just attached the zipped log.

[socratis]

This is the only thing that I saw worth mentioning in your log:

00:00:02.855178 VRamSize <integer> = 0x0000000001000000 (16 777 216, 16 MB)

Take that to 64 or 128 MB. And try to toggle the 3D acceleration, maybe enabled is the way to go...

[ordinarysoul]

It worked after updating VRamSize to 64MB. Thank you for your help!

[socratis]

Great, thanks for the feedback. Marking as [Solved].