As I was trying out different chess applications I received a hint from the Lucas Chess virtual tutor, showing me I had a better option: Move piece C2 to C1. When I did that, the piece promoted to queen. I've tried this on chess.com with the same result. What (strange) rule is this, and where can I learn more about promotions?
EDIT: Or, maybe this got triggered by the fact I had a crashed play, and had set-up the board manual, making a mistake by flipping the board so the computer thinks I'm already on the other side. That must be it, right? :)