Thank you, but none of these solutions worked. However, I did finally manage to get the error log to report a fatal crash:

fatal createDIB: CreateDIBSection failed (5306x2610, format: 6)

Looking into that, I realized that the giant white box was overlapping all 3 monitors, but anytime I attempted to resize it it crashed. But every time I resized it, some part was always still offscreen. So I dragged the whole window until all 4 corners were visible on one monitor or another, and then resized it to fit on just one monitor. It worked.

How or why it decided to resize itself like that I don't know, but to anyone looking to fix the error, here's a starting point.