The sleep and hibernate features are disabled when Hyper-V is installed on a Server 2008 (R2). This is by design.
Lets start a long journey to find a workaround ..
powercfg.exe /hibernate on
Disable Hyper-V Service?
I stumbled upon this blog post: http://old.markharrison.co.uk/blog/2008/09/sleep-hibernate-with-hyper-v.htm
You can have Hyper-V installed but the service not started by the following registry setting:
Windows Registry Editor Version 5.00
When the computer starts, all power management features are available.
If you need a VM (for a demo for example), Hyper-V can be started with the following command:
net start hvboot
Power management will be disabled again, until the next reboot.
As I don’t need Hyper-V most of the time this is the workaround for now! The journey wasn’t that long after all!