@bitinn Turned out a Common UI widget was blocking input for the entire game, because it was initialized as active, and it took us two days to find out because it was also initialized as invisible, which is not the same thing.
The underlying problem is that Unreal is a collection of autonomous but heavily interconnected systems, so setting anything up correctly means looking into several unrelated places. It's particularly bad with the UI, but other modules have this issue, too.