Thanks, calvinwill, for your help, and for the very helpful photos of your gearbox. You're right: there's no denying the unpleasant fact that I have to open the whole thing up. I will try to do that in the next couple of days and take a photograph for posting if I can't figure out the problem myself. But I was so careful with every part, every gear, that I can't imagine that I omitted anything. We'll find out soon enough.

Many, thanks. You guys are great.