Apparently, the board had developed a sprite error during transport. We arranged for me to have a look at it again, and a few weeks after New Year, I had the board back on my test bench again. I was pretty sure of what was wrong: The crappy sockets that Konami had used for the ROMs. And checking with the continuity tester confirmed my hunch. So I desoldered all the 6 sockets for the graphics ROMs
and replaced them with (as I always use) machined pin breakable header strips
And now, the sprite error is gone
After having tested the game several times and flexed the board, I'm now hoping that it'll reach it's rightful owner (again) with no additional errors.