at first, thanks for the great response here...
i found the reason for this bug, so this is maybe interessting for the arturia staff and something to handle for future versions.
in fl-studio you can "detach" windows, for example to move them to a second screen. i usually run spark in a detached window on my second screen and thats the reason.
if spark is running "detached" then the submenus wont work (its impossible to route mixer tracks this way). you can open a sub menu, but as soon if you move the mouse its gone. this problem comes with spark 1.7, in former versions i never experienced this problem.