Linux guest shuts down when I lock Windows 8.1 screen

[illetserd]

Hello;

I am running a 64-bit Ubuntu Linux 12.04 guest on 64-bit Windows 8.1 host. My Virtualbox version is 4.3.8 r92456. The guest is running the guest additions, also version 4.3.8. I run the guest in seamless mode.

The host is my primary windows workstation so when I step away from it I lock the screen with the Ctrl-Alt-Del followed by "Enter" sequence. This locks the display. Infrequently, maybe 1 time in 20, the Linux guest shuts down. The last log messages are shown below:

07:39:49.690751 Guest Additions capability report: (0x5 -> 0x4) seamless: no, hostWindowMapping: no, graphics: yes
07:39:49.691065 UIMachineLogicSeamless: Leaving 'seamless' as it is no more supported...
07:39:49.770101 Display::handleDisplayResize(): uScreenId = 0, pvVRAM=0000000007ab0000 w=1920 h=1040 bpp=32 cbLine=0x1E00, flags=0x1
07:39:49.770250 Display::handleDisplayResize(): uScreenId = 0, pvVRAM=0000000007ab0000 w=1920 h=1040 bpp=32 cbLine=0x1E00, flags=0x1
07:39:49.844131 Display::handleDisplayResize(): uScreenId = 0, pvVRAM=0000000007ab0000 w=1920 h=1040 bpp=32 cbLine=0x1E00, flags=0x1
07:39:49.848606 VMMDev::SetVideoModeHint: got a video mode hint (1584x779x0)@(0x0),(1;0) at 0
07:39:49.858144 Guest Additions capability report: (0x4 -> 0x4) seamless: no, hostWindowMapping: no, graphics: yes
07:39:49.858233 VBVA_INFO_SCREEN: [0] @0,0 1920x1040, line 0x1e00, BPP 32, flags 0x1
07:39:49.858270 Guest Additions capability report: (0x4 -> 0x0) seamless: no, hostWindowMapping: no, graphics: no
07:39:49.993425 UIMediumEnumerator: Machine (or snapshot) event received, ID = dedb0e5e-2877-49c5-9a8f-baa1c28af133
07:39:49.993471 UIMediumEnumerator: Old usage: 08c8038d-6e06-40cd-a4df-c976e59a3c43, 0f0bc695-62e7-4a6e-98c1-76f7c83e5209, 175f9ebb-4d1e-40fb-b1f0-fb5a5a5c04ea
07:39:49.998032 UIMediumEnumerator: New usage: 175f9ebb-4d1e-40fb-b1f0-fb5a5a5c04ea, 0f0bc695-62e7-4a6e-98c1-76f7c83e5209, 08c8038d-6e06-40cd-a4df-c976e59a3c43
07:39:49.998063 UIMediumEnumerator: Machine (or snapshot) event processed, ID = dedb0e5e-2877-49c5-9a8f-baa1c28af133
07:39:50.012591 Display::handleDisplayResize(): uScreenId = 0, pvVRAM=0000000007ab0000 w=640 h=480 bpp=32 cbLine=0xA00, flags=0x1
07:39:51.154319 Guest Additions capability report: (0x0 -> 0x4) seamless: no, hostWindowMapping: no, graphics: yes
07:39:51.155147 VMMDev::SetVideoModeHint: got a video mode hint (1584x779x0)@(0x0),(1;0) at 0
07:39:51.180107 Guest Additions capability report: (0x4 -> 0x4) seamless: no, hostWindowMapping: no, graphics: yes
07:39:51.180198 VBVA_INFO_SCREEN: [0] @0,0 1920x1040, line 0x1e00, BPP 32, flags 0x1
07:39:51.180285 Display::handleDisplayResize(): uScreenId = 0, pvVRAM=0000000007ab0000 w=1920 h=1040 bpp=32 cbLine=0x1E00, flags=0x1
07:39:51.202364 Guest Additions capability report: (0x4 -> 0x4) seamless: no, hostWindowMapping: no, graphics: yes
07:39:51.202445 VBVA_INFO_SCREEN: [0] @0,0 1920x1040, line 0x1e00, BPP 32, flags 0x1
07:39:51.221854 Display::handleDisplayResize(): uScreenId = 0, pvVRAM=0000000007ab0000 w=1920 h=1040 bpp=32 cbLine=0x1E00, flags=0x1
07:39:53.037659 Guest Additions capability report: (0x4 -> 0x4) seamless: no, hostWindowMapping: no, graphics: yes
07:39:53.037746 VBVA_INFO_SCREEN: [0] @0,0 1920x1040, line 0x1e00, BPP 32, flags 0x1
07:39:53.037785 Guest Additions capability report: (0x4 -> 0x0) seamless: no, hostWindowMapping: no, graphics: no
07:39:53.066987 Display::handleDisplayResize(): uScreenId = 0, pvVRAM=0000000007ab0000 w=640 h=480 bpp=32 cbLine=0xA00, flags=0x1
07:40:08.708428 OHCI: Software reset
07:40:08.839078 Entering S5 power state (power down)
07:40:08.839141 Changing the VM state from 'RUNNING' to 'POWERING_OFF'.

I locked my screen at 07:39:49. The Guest Additions seem to go through some machinations then the machine is powered off. At most other times, I can lock and unlock the host screen without issues.

This happens whether or not 2D graphics acceleration is enabled or not.

I have attached the complete VirtualBox log. Has anyone encountered or, better yet, solved this issue?