Okay, problem solved when I installed a completely different piece of software, possibly because it installed the Microsoft Visual C++ 2017 Redistributables, which must somehow have gotten deleted? After that I was able to redownload and reinstall Polyphone again and it worked. (Online advice about this error message did suggest removing and reinstalling these redistributables, and I'd tried it, but the 2017 version wasn't there to be removed at that point and I didn't think of *adding* it. I must have uninstalled something else that took the redistributable package with it, and I didn't realize. Or at least that's my best guess right now, and it could be completely wrong. Polyphone works now anyway.