@brunob: What I meant is that I can confirm that the icon is missing in the treebrowser panel but it is showing in the filebrowser panel.
I am using Ubuntu 16.04, gnome desktop, standard theme.
@codebrainz: what I wonder about is, that it works in one plugin but not in the other. The icon itself seems to be present in the theme otherwise it would be missing in all plugins. Or am I overseeing something?