PhonePayCheck problem

I have been using Phonepaycheck for years now and like it. I have 2 phones running it, my old Samsung Galaxy Core and my new Samsung Galaxy S7. Both have been running fine until today. On my newer phone, the Galaxy S7 i now get the message "Sign-in failed: Authentication rejected." "Try again." Each time I click on "Try Again" I get the same message.

Does anyone know what I should do to fix this? Thanks for your help.