Hi Amanda,
thanks for your answer. I already had it that way but still got the error message.
Now I know what the problem was: Boost ist using the private function build_action_menu_from_navigation() (as written in the error message... ;) ) and this could, of course, not be called. I only had to copy this single function into my child theme core_renderer.php to get it working.
I still think that this is suboptimal, but still better than copying all functions.
Thanks and cheers, Kathrin