When dealing with checkboxes, the #title that you use is sometimes not very good to be used with error messages. In this case you can add the '#element_title' to your element and it will replace the this title with so the original title can be used for the error messages

example.module

/**
 * Implements hook_preprocess_HOOK().
 *
 * HOOK: 'form_element'
 */
function example_preprocess_form_element(&$variables) {
  if (isset($variables['element']['#element_title'])) {
    $variables['label']['#title'] = $variables['element']['#element_title'];
  }
}