Also fixed on my end. It turned out to be a 3rd-party license. It seems like Korneff Audio’s licensing system was affected by the last Windows update. Since I have their limiter plugin as part of my production templates, the licensing system fooled me into believing it was Cubase who was complaining about the license (since there was no application title on the licensing window). I only stumbled onto it because I recently had to work in Ableton Live, and the same licensing window came up when I dragged an instace of the limiter into a track. That clued me into what was going on. After re-authorizing the plugin in Ableton, the window stopped showing up in Cubase.
My only question is why Ableton was able to successfully register the new license for the plugin, whereas Cubase couldn’t. Anyway… issue resolved, and now I know what to look for in the future.