@77_Scout: Exiting the mission brings up the "This flight will not be logged..." (or words to that effect) message and sends me to the debrief screen.

The cloner program ran and seemed to finish correctly (78 of 78).

I took my time and did the whole procedure by the book, twice (including installing it on a second admin account). Sadly, no change.
I'm about ready for a stiff drink now, but it's already well past my bedtime.

Will do another clean install tomorrow, then gather together all the logs and send them off.

Thanks for the advice, gents!