Got it :-) Just manage to install gstreamer from latest sources with
git clone <https://gitlab.freedesktop.org/gstreamer/gstreamer.git> and the ninja/meson stuff. (see <https://discourse.gnome.org/t/installing-gtk4-for-testing-on-opt-ii/3349/4>) And I finally get the same error as you get. Will investigate and fix it in the next days.
