Windows 8.1 Guest Keeps Rebooting after 5.1.8 upgrade

[virtual peter]

Hi,
I've upgraded last Friday to 5.1.8 and a Windows 8.1 guest that i have, started to reboot randomly...
It boots correctly but as soon as i log in, after a random period (never bigger then 2 minutes) it crashes with "... your pc ran into a problem..."
Some notes:
- The guest vm was working perfectly until 5.1.6, and the problem started after upgrading to 5.1.8.
- The installed extensions are 5.1.8 related.
- The guest vm has 6G RAM (it did work perfectly with 4G)
- The guest vw is only used for SQL Server Database small tests

What I've done so far:
- Tried changing the settings of the vm (ram, usb, etc) that do not trigger reactivation: did not solve the problem
- Downgraded to 5.1.6: did solve the problem, but i don't like the solution

Thanks for any help,
Peter

[socratis]

1. Start the VM. Not from a saved or paused state. Clean start.
2. Take the steps required to generate/observe the error. Record the error message. EXACTLY. Post a screenshot if you have to.
3. Shut down the VM (if it hasn't aborted by itself). Not saved, not paused. Complete shut down. If you can't shut it down by normal means, close the VM window and select "Power off".
4. Right-click on the VM in the VirtualBox Manager. Select "Show Log..."
5. Save it (just the first log), ZIP it and attach it in your response (see the "Upload attachment" at the bottom of the form).