iPhone 15 Pro Max no longer runs Mayo Clinic app

I installed the Mayo Clinic app about a month ago, and at first it worked fine. Then, while I was at the clinic, it stopped working and still doesn't work. The welcome page opens, and then a login page follows, which asks for username and password. When I enter my credentials, nothing happens. I can log in from a url on Microsoft Edge or Safari on my iPhone and get my data that way, but not from the app. I've deleted it and reinstalled it, made sure my OS and the app are up to date, etc.

Having a similar problem with American Airlines' app. When I press "log in," I get the response, "something went wrong," but I can get to my account from the url "aa.com." I'm not using a VPN, but am using Avira Security Pro antivirus software on the phone.