I just upgraded to FW 21.19 a few minutes ago. At the "end" it restarted itself, I saw the blue rectangle. Then a few minutes later when I looked at it again I only saw: "Cannot Unlock Maps". And it's stuck there ever since.
update1: I still don't know if the solution provided by Garmin-Sierra below works, but I can tell you: only start upgrading to 21.19 if you have a computer with Garmin Express on hand and a few hours to waste sitting and waiting for GE to download maps, all while you can't really do anything else, 'cause you have to click, connect, disconnect the watch...
update2: It took a while. At the end reinstalling only the maps did not fix the problem. However then I looked closer to the error messages (again I can't upload images today...):
1. Cannot Unlock Maps
2. Verifying Connect IQ Apps
3. Content Expired - Update your downloads to continue offline use
So then I realized that the problem was not only with the maps, but also the offline music I downloaded in the past in the Spotify app. So I opened Spotify, updated the playlists, and voila, after the next reboot I had no longer errors.