title: Failed user login on symfony 3.4
body: <p>I'm trying to build a login system that checks my database <code>User</code> entity in order to authenticate.</p>
  
  <p>My <code>security.yaml</code>:</p>
  
  <pre><code>providers:
      db_web:
        entity:
          class: AppBundle:User
          property: username
  </code></pre>
  
  <p>My <code>loginController</code> function is from the documentation:</p>
  
  <pre><code>public function login(Request $request, AuthenticationUtils $authenticationUtils)
  {
      // get the login error if there is one
      $error = $authenticationUtils-&gt;getLastAuthenticationError();
  
      // last username entered by the user
      $lastUsername = $authenticationUtils-&gt;getLastUsername();
  
      return $this-&gt;render('/login/index.html.twig', array(
          'last_username' =&gt; $lastUsername,
          'error'         =&gt; $error,
      ));
  }
  </code></pre>
  
  <p>and the error I'm getting:</p>
  
  <blockquote>
    <p>Authentication request could not be processed due to a system problem.</p>
  </blockquote>