It's really strange, but when S+ is active, all selectboxes on any page in Opera browser stop working. When I click on any, it won't open. Strange is, that they can't be opened even by keyboard. I've tried to add Opera into Ignored list but it didn't help. When I disable S+ (by left-click on tray icon), selectboxes are ok.
Try unchecking Keep Gesture Window On Top* and Don't Hide Gesture Draw Window*. The gesture drawing surface is a full-screen transparent window. It's possible that Opera thinks it's an application window and the select boxes don't react if there's a window above them. Those settings have a small effect on performance, but only noticeable on low-powered systems.