If the Mac app is not loading correctly then it might require a bit of surgery to remove the app cache.
Quit the App
You need to show 'invisible files'; on your Mac. this is easy to do. Just hold down these 3 keys together: Command Shift Period (.)
once you do that you can see the invisible files on your mac.
Then go to this location:
Users/ [your USERNAME ]/Library/Containers/
Double Click / Open the Containers folder and scroll down inside. You will see a folder called NoteDex
MOVE THIS FOLDER TO TRASH
Now you can relaunch the app and the login screen will appear. Login again as usual. Your data will sync down from the cloud and you'll be all set to go in minute or two once the data syncs.