Force Reboot in InstallScript MSI project

We have a InstallScript MSI project.
For a specific functionality of our software it's necessary to add a windows feature on Server 2008 R2 / Server 2012 (Feature DesktopExperience). This feature needs a reboot after getting installed.

I'm activating this feature with pkgmgr.exe /quite... in order that the computer will not reboot immediately. Instead of that, the installation should be finished and the user is asked for a reboot.

For this I have set the InstallScript variable BATCH_INSTALL to „true“ and the SdFinishReboot dialog is shown as expected. But when I select "Yes, reboot now" the installation is finished, but the server doesn't reboot as he should.

I have checked the result of call System(SYS_BOOTMACHINE). The return value is 6 (what means WILL_REBOOT), but there is no reboot at all.

Is this perhaps a permission issue on Server OS?
Nevertheless as soon as I start the setup with the property "REBOOT=force" a standard reboot dialog is shown and the machine reboots as expected.

How can I force a reboot on user selection with InstallScript project on Server 2008?

Thanks for your help.
Thomas