Just encountered this exact same issue today. Was trying to walk my Dad through the process and thought he was just doing something wrong. But I had him send me a screenshot of the iPad screen and it is exactly how you described. I had him delete then reinstall the app, had him hard reset the iPad, his iOS is fully up to date. I went to my iPad and was able to get in without any issue. That's what's the most frustrating part, it seems specific to his device (and yours). He has a slightly older iPad (iPad 3, I think) and I have an iPad Air. But the iOS versions are the same and fully updated.
Have you figured anything out?
I hope my screenshots show up ok. I had to drag and drop them in because the option to insert photo was grayed out for me on here. (I blurred out the front part of my email for privacy)