POSSIBLE SOLUTION: (Or at least treating the "symptoms")
I noticed sometimes my iPad would actually start to connect, and then it would immediately drop, as if the router was doing some kind of dynamic switching that was knocking it off. I experimented with several dynamic switching options in my router configuration and finally had success when turning off what my ActionTec router calls SmartSteering — the configuration that lets it switch clients dynamically back and forth between 2.4G and 5G.
I turned this off, which created separate MyNetwork-2.4G and MyNetwork-5G wifi networks. And then, what do you know: connecting my iPad to the 2.4G network works great! And connecting it to the 5G network (with the exact same password) fails every time with that "incorrect password" prompt. So at least in my case, it was a "connect to 5G" problem disguised as a password error. Worth a try.