We implemented "Login with Amazon" functionality and everything works fine for a long period of a time.
But a few days ago we moved our site to HTTPS and we noticed that a user can't login anymore via Amazon Login. User always get a message "There was a problem. We can not find an account with that email address". But if we use same credentials at Amazon.com, everything works find and user can log-in.
We checked settings and everything looks correct, firstly we thought that may be problem is connected with our migration to HTTPS, but we double-checked integration and we didn't see any issues with moving to HTTPs.
I have visited your site and I am able to reproduce the issue and see the account not found error.
It appears that your site is integrated with Amazon Pay and you were performing testing in Sandbox mode. Amazon Pay Sandbox requires special test accounts to be used, meaning that accounts created on Amazon.com cannot be used; this is why you are seeing the error message.