It turns out my copy of Downie 4 was corrupted. (I haven't used it for awhile so don't know if it was during this macOS update or sometime in the past. I went to my Time Machine backup and restored it from before today's update but it still wouldn't launch. So I just deleted the app and went back to the developer's site to get a fresh download. I have installed that, re-input my license key and all is good again.