virtualbox.org

Hello everyone
I have installed several virtual machine on my Windows 10 host, and they work just fine.
After I updated the VB to 6.1.12, and installed the VB extension pack my virtual Windows 8.1 doesn't boot (Windows 10, Ubuntu and Windows Server work).
When I power off the machine and try to start it again the Guru meditation shows up (only after 3-5 unsuccesful tries to start the Windows 8.1).

What should I do?

This log does not show a guru meditation:

00:00:09.891903 VMMDev: Guest Log: BIOS: Booting from Hard Disk...
00:00:09.936411 Display::i_handleDisplayResize: uScreenId=0 pvVRAM=0000000000000000 w=720 h=400 bpp=0 cbLine=0x0 flags=0x0 origin=0,0
00:00:42.924276 Changing the VM state from 'RUNNING' to 'SUSPENDING'

It appears to turn off the display, if I read this right, then the guest windows is closed 33 seconds later.

Please try uninstalling the Extension Pack and Virtualbox, and install 6.1.10 Virtualbox. Skip the EP for now. Does the guest boot?

Well, these logs are saved when Win didn't boot, but as I said it takes 3-5 tries to show the Guru.
Before I try your advice should I SAVE my guest files and where?

P.S. I'm not very familiar with Virtual Box , even though I use virtual machines on it

Your guest's files will live through the downgrade, just like Word's documents live through a Word version change.

The purpose of the downgrade is to try a version that didn't have lots of (or any) reports about the screen turning off.