Gtk 2 to be phased out...
It is about 10 years that gtk2 has been put on hold to develop gtk3 (and now gtk4). Distribution are talking about dropping gtk2 support (notably fedora). And this starts to be visible. I'm attaching a screen dump obtained on fedora 34 (on 33 it was fine).
It starts to be urgent to port OpenViBE to a newer version (or another toolkit like Qt, which would be even better but this is much more work).