I found the reason myself (as very often nachoparkers hint was very helpful):
I installed in the VM a pure Debian from the ISO image. This image had the linux kernel version 4.9.0-8-amd64. In order to get a better support for SSD’s in BTRFS I installed a more recent kernel from the backports: 4.17.0-0. (The version 14.0.4 is not available in the Debian Stretch backports anymore, unfortunately.)
And voila: The NCP Web Panel starts and displays the Info page just nicely!