Ok my previous search did not show that this bug is well known. Here is the solution.
https://support.plesk.com/hc/en-us/articles/360009765740--FIXED-BUG-Cannot-change-the-password-via-webmail-interface-for-domains-in-Plesk
also mentioned in this article...
Hello @AlL,
System:
Debian 9.12
Plesk Obisidian 18.0.23
Mail: Postfix + Dovecot
Webmail: Roundcube
i have a question. How can users change there passwords for mailservice independetely from an administrator with plesk access? In the described situation users a not Plesk users but normal people...