I wrote an authentication plugin for Joomla 3.4.5. After logging in (with no other auth-plugins enabled) I got this message:

You can't access the private section of this site.

First I thought this was a problem of my plugin, so I changed it to the following:

 public function onUserAuthenticate($credentials, $options, & $response)
 {
      $response->error_message = '';
      $response->fullname = 'Max Mustermann';
      $response->email  = 'mymail@domain.tld';
      $response->username = 'username';
      $response->status = JAuthentication::STATUS_SUCCESS;
 }

So actually I should be authenticated immediately. But I still get the error message from above.

It's a absolutely fresh installation of Joomla with two main entries - one is set to "Public", the other one to "Registered".

Why can't I login?