Just downloaded the app again, and there isn't a readme.txt file with it.
Extracted the contents of the Setup.msi, and found the following line in the WallpaperFlickrSettings.xml
Perhaps a new apikey needs to be obtained (from Flickr) by each user, and then we can keep using it?
I suspect there may have been a sudden surge in popularity for this app, and now Flickr's shut down the existing API key.
Good luck to anyone else who's trying to do this, too.