This is the intended behaviour - as long as any client is active the server has to be up, too.Found my issue.
Multiseat and PowerScheduler++ "knows" that the clients are there (even although the are nOT using the HTPC, so it prevents the Windows Hibernate request from working.
I proved this by disconnecting my clients from the network. Voila, HTPC goes into hibernation.
Hope yours turns out to be something as silly as this.
Michael