libvirt users - Jun 2018 - Are more then one overlay possible without recieving a "Permission denied"?

I've created a backing chain like this. FirstFollr.ovl is an overlay to
Base.img and so on.

    Base.img        (ok - Win 10 Installation starting)
    FirstFloor.ovl  (ok - Win 10 Installation starting)
    SecondFloor.ovl (permission denied -> Base.img) => assumed Bug
    Roof.ovl        (permission denied -> Base.img) => assumed Bug

Using virsh / virt-manager, I can only start Base and FirstFloor. Starting
Second and Roof are failing.
What's wrong?

Details are described in:
https://bugzilla.redhat.com/show_bug.cgi?id=1588373

System infos:

    Compiled against library: libvirt 4.0.0
    Using library: libvirt 4.0.0
    Using API: QEMU 4.0.0
    Running hypervisor: QEMU 2.11.1

    Ubuntu 18.04 LTS (Dell Precission Tower 3620)