Mailuser password malformed when using a umlaut

Summary

Letters with a umlaut in a password for a mailuser, like ä or Ö are malformed. The user can not log in.

Steps to reproduce

1. Set the password to "ällo3456"
2. Try logging in.

Correct behaviour

The user should be able to log in.

Environment

Server OS + version: Debian 10
ISPConfig version: 3.2.4

Software version of the related software: