Older versions of iOS (up to version 13) are not affected. The error lies with both Apple iOS and Microsoft Exchange Server. Unfortunately, neither company has yet fixed the faulty behaviour.
Background: From mobile devices, in general, only the personal mailbox can be accessed (this is a limitation of the ActiveSync protocol used). In order to be able to at least process the e-mails from a functional mailbox, this can be integrated via IMAP protocol. Unfortunately, this is currently also not possible under iOS; the login is not accepted when accessing the function mailbox.
Until a solution is found by Apple and/or Microsoft, you can access function mailboxes from iOS devices using the groupware web interface (https://mail.tu-darmstadt.de/).
The HRZ is looking for further alternatives for access.