I'd be surprised if that's the only thing that isn't working with GIMP, when S+ is active.
I found quite some time ago that GIMP and S+ don't really get along, and on my machine I have GIMP on the S+ Ignore list, with the disable S+ if application active selected.
Back when I did this, the ignore list on it's own wasn't enough and the checkbox selection was required, that may have changed as I haven't tested for quite some time but it was the only way to get certain things in GIMP working correctly again. I can't remember exactly what the issue was for me however, but I think it was something to do with selecting functions in the panels with the mouse button.