I just noticed that the errors that I see only show up when I run Gramps in Dutch. When I switch to US English, like you do, they disappear, but I still don’t get the GTK errors reported by you. And that is quite weird, because contrary to Linux, where GTK comes with the OS, every Windows install has GTK built in, so your GTK should be the same as mine.
Do you have anything installed that could mess with your GTK, like other GTK based applications, or non standard themes? I’m on my 3rd Windows config now, and I can’t repeat anything.
Or should I dig up a wireless mouse from my attic?