Users can't change password

Users can log in to the UCS portal, but they can not change their passwords (using the “hamburger” menu in the upper right corner)
Error Msg:

Error:
Change password

Changing password failed. The reason could not be determined. In case it helps, the raw error message will be displayed: Errorcode 20: The new password could not be set.

Other forum posts on the same topic are of little help to me. My UCS installation is stand-alone, e.g. no replication, AD integration etc

From management-console-server.log:

05.07.22 19:01:46.882 LOCALE ( WARN ) : Could not find translation file: ‘umc-core.mo’
05.07.22 19:01:46.902 MODULE ( PROCESS ) : Setting auth type to None
05.07.22 19:01:46.920 LOCALE ( WARN ) : Could not find translation file: ‘umc-core.mo’
05.07.22 19:01:47.645 MAIN ( PROCESS ) : LDAP bind for user ‘uid=volker,cn=users,dc=fr,dc=atrics,dc=com’.
05.07.22 19:02:05.907 AUTH ( WARN ) : Changing password failed ((‘Authentication token manipulation error’, 20)). Prompts: [('Current Kerberos password: ', 1)]

Any help?
Regards