oh, i think i have set this setting
Use Tiki authentication for Admin log-in
If this option is set, the user “admin” will be authenticated by only using Tiki’s user database and not via LDAP. This option has no effect on users other than “admin”.
but i have no user "admin", only a user "user". I have thought that it means admin-users as a group, not a single user name. Because there was no "admin" user by default only a "user" user (which had admin rights), i thought it was just a unclear documentation.
Can i set it back via https://doc.tiki.org/Preferences---Log-in ?