On an existing Drupal system I see a strange phenomenon; user names disappear from the admin/people page. I've disabled the realname module but still it happens. How to debug such an issue?

Edit: Additional Information

My admin theme is Seven, so I've added a code to print the options array:

function seven_form_user_admin_account_alter(&$form, &$form_state, $form_id) {
  dpm($form['accounts']['#options']);
}

The result: The username fields contains the following typical information:

<a href="/user/322" title="View user profile." class="username" 
xml:lang="" about="/user/322" typeof="sioc:UserAccount" 
property="foaf:name" datatype=""> </a>