You are using Safari in private mode. This means it is not allowed to tell the app that you are logged in. Turn off the private mode and try again.
Something is wrong with your cookies and internet history. Unfortunately, we have not yet been able be more specific. But if you clear your cookies and internet history in Safari mobile, you will be able to log into the app again.
If neither of the above points help, install another browser on your iPhone, for example Chrome. This will allow you to log in. Or do not use the app, but alternatively my.rotterdamuas.com on your phone, if you do not want to delete your history, or do not want or are unable to switch browsers.
Notification: "Session not found", now what?
This occurs with the iOS app in combination with Safari for mobile and, unfortunately, we cannot solve it with the app. It may have to do with the following things: