Thanks for your answer.
I renamed the file, it is already on SQL, which should be right for me. (MySQL database)
Nevertheless, I get the same error message again when trying to change the password.
In the error log you can read about it:
[07-Jun-2019 09:45:23 +0200]: <bff126ec> DB Error: [1305] FUNCTION d02eaa95.update_passwd does not exist (SQL Query: SELECT update_passwd('$1$S2ftez0T$R4eOA58mvJ13.SCSjoEby0', 'kassenwart@domain.de')) in /www/htdocs/w018i29c/domain.de/roundcubemail-1.3.9/program/lib/Roundcube/rcube_db.php on line 543 (POST /?_task=settings&_action=plugin.password-save)