Over night - so without changing anything - the problem got worse. After choosing the merchant in the login procedure, I get an orange Error box and the Clover logo in gray symbolizes loading forever. I have attached the console output of Chrome, hopefully this helps to clarify what is going on here.
Thank you for bringing this to our attention. There was an issue with OAuth 6 days ago but it has since been resolved. Can you please provide (in a private comment) your app ID so that I can investigate further? It'll also be helpful if you can post a code snippet of how you're handling authentication on your web app.