When you send your physical computer running Windows 8.1 to sleep (stand-by), the computer screen goes black but all other power lights remain on, and you cannot wake it up by pressing any key or whatever, you have to hard reset it by pressing the power button till it stops.
The main problem in my case at least was a conflict between a VirtualBox network driver that it installed on Windows 8.1 (the host) to support “Bridge” networks in virtual machines (clients). Windows hangs at trying to turn this off.
Note that it’s not the same device you see in front of other network adapters in device managers. The one you see is the “host only network”.
I managed to solve the problem by uninstalling VirtualBox, and then installing it again without Bridge network support. In the install wizzard “Custom Setup” step, under “VirtualBox Application” -> “VirtualBox Networking”, I disabled “VirtualBox Bridged Networking”, and continued the setup with everything else set (the default).
Note that I tried removing bridged networking from the virtual machines that used it and it didn’t solve it. I had to remove the feature as explained above.
But I need bridged networking!
Ideally, most of the use cases for bridged networking will be solvable using NAT networking instead, use this for your virtual machines unless you really really need the bridging, and then you need to choose between it and the ability to set the computer to sleep, at least until BirtualBox solves the problem.
Applies To: (Version information)
I experienced this problem with VirtualBox 4.2.18. Most likely the problem itself will be solved in some future version.
If you want to watch for progress on this issue, check out the issue report in VirtualBox issue tracker: