This is a known and tricky issue were are actively investigating on. At the moment, it does not appear to be an iOS app problem, though. It only surfaces there most of the time. For still unclear reasons it appears like the server has a problem in the specific situation of iOS clients attempting to log in. The web view in the iOS app login is simple and only follows the instruction of the server.
There are other forum threads about this already and we also have a GitHub issue which tracks this problem.
As a workaround, you may create an app password in the web user interface and scan the QR code with your app which circumvents the web-based client login and should work immediately without issues.