Windows Server 2012 guest agent won't install

[linum]

I enabled qemu guest agent support with "qm set 170 -agent true" and verified that the agent option is used (kvm is running with -device virtserialport,chardev=qga0,name=org.qemu.guest_agent.0). So I assume the host part is ok. But if I try to install the client side (the qemu-ga-x64.msi from 1.6.8 iso) I get an error:

Service `QEMU Guest Agent? (QEMU-GA) failed to start. Verify that you have sufficient privileges to start system services.

Since I've launched the msi Installation from an administrator prompt insufficient rights shouldn't be the problem. Leaving the error dialog still "online" I moved to "C:\Program Files\qemu-ga" and run the qemu-ga.exe with the -v option and get the following messages:

<timestamp>: critical: error opening path
<timestamp>: critical: error opening channel
<timestamp>: critical: failed to create guest agent channel
<timestamp>: critical: failed to initialize guest agent channel

Does anyone know what's wrong?