Thanks for the quick replies.
Yes, I did build the mine, and have more than 100 crystals handy on the savegame IIRC (in any case definitely more than 25 ). The spawning of the farming patch, the mine and the pioneer all happen correctly in my game.
I went back to a previous savegame yesterday. This was after getting the quest, but before defeating the ogre. I had the same issue after collecting 50 crystals, leaving Relias in town the whole time, building a caravan, etc. Note however that I initially got the problem WITHOUT loading a game (i.e. I saved the game before the ogre battle, but I didn't reload it). I'll have a look at my other savegames, but I think to get to a point where I didn't receive the quest, I'd have to use the "end of chapter" saves, which would take me an hour back or so.