It has been 40 hours and nobody claims to have found the bug, so I reported it to Malafador. I guess nobody was trying, because with all the hints it should have been easy to find. Possibly it is something peculiar about my game, the race I was trading with, or the tech exchanged. In any case it should take about five minutes to fix, so I expect it will be in the next patch. In other words, if you have been looking for it, don't bother. What I did would be a game exploit even if you knew what it was.....