Dear Colleagues,
There is a known bug in the latest stable VirtualBox release that causes virtual machines to crash on startup when using a Windows host machine:
https://forums.virtualbox.org/viewtopic.php?f=6&t=62615
Seems that sometimes this can be circumvented by disabling antivirus, or by updating to the test release linked in the above thread (4.3.15). For me, neither of these solutions worked, and I had to downgrade to version 4.3.12 (https://www.virtualbox.org/wiki/Download_Old_Builds_4_3)
Regards, ilia