Can't unlock the lock at the user tab when 'Local Network Users' is selected

At our office we use the mac mini server with the server.app software. To manage network accounts for all iMacs, this way we can all setup our own desktop environment to what you personally prefer. All this works fine, it even doesn't limit you to use only one specific iMac because it will sync your account to the server.

Unfortunately something has changed in the server.app, we now can't add new nor remove old user any more. When adding a new local network user at the "Users" tab with "Local Network Users" selected in the drop down list on top. Than there is a (new?) lock shown next to the add and remove (+ and -) sign, see the screen shot below.

When clicking on it a pop-up is shown, what asks for an administrator name and password. Credentials we don't have or never have registered. Also I noticed that the credentials have something to do with the LDAP (because of /LDAPv3/127.0.0.1 no idea what it means), this is what we turned off... Have a look, comment if you need more information or specifications.