I am posting this in case anyone else has seen this behaviour.
I have had reports from several users today that their app (running in a browser) briefly opens and then exits, returning them to the Power Apps Home page. Some users also use the app on a phone, but report that this still works.
The app itself has not been changed for over a week and is used by about 200 users, often several times a day.
The same behaviour occurred at the beginning of June, again affecting several users. After four or five days, the problem then stopped (again without my changing the app).
I cannot reproduce the error but the users are getting it consistently. They have tried using an InPrivate browser session, but that made no difference. The fact that I have five users reporting this within a few hours makes me wonder about a Microsoft Edge change, or a Power Apps browser code update. I am waiting for feedback from the users about whether it works on another browser.