Ok, I think I've narrowed down the problem, and I don't think I really needed to disable mods. I ended up transferring Jeb from the lander can to one of the orange inflatable pods. I then transferred a tourist from their USI Life Support cupola to the stock lander can, lifted the craft off the surface, then landed again. That tourist, and ONLY that tourist, then completed "land on Minmus." So it seems to be a problem with USI Life Support parts.