How to resolve Recoverable fatal error: Object of class Drupal\Core\Link could not be converted to string in Drupal\Component\Utility\Xss::filter?

I'm following the How to Configure SimpleSAMLphp for Drupal 8 on Acquia instruction. I'm at the bottom where it says, "SimpleSAMLphp_auth module settings. I personally recommend to store configuration for SimpleSAMLphp_auth module settings in settings.php." Once I copied the code he has in that code snippet to my settings.php file (pasted it at the bottom) and push it to Acquia, I got this error when I tried to login via the dev.mysite.com/user url.

The website encountered an unexpected error. Please try again later. Recoverable fatal error: Object of class Drupal\Core\Link could not be converted to string in Drupal\Component\Utility\Xss::filter() (line 67 of core/lib/Drupal/Component/Utility/Xss.php).

The code shown below is what I have in my settings.php file.

$config['simplesamlphp_auth.settings'] = [
  // Basic settings.
  'activate'                => TRUE, // Enable or Disable SAML login.
  'auth_source'             => 'default-sp',
  'login_link_display_name' => 'Login with your SSO account',
  'register_users'          => TRUE,
  'debug'                   => FALSE,
  // Local authentication.
  'allow' => [
    'default_login'         => TRUE,
    'set_drupal_pwd'        => TRUE,
    'default_login_users'   => '',
    'default_login_roles'   => [
      'authenticated' => FALSE,
      'administrator' => 'administrator',
    ],
  ],
  'logout_goto_url'         => '',
  // User info and syncing.
  // `unique_id` is specified in Transient format, otherwise this should be `UPN`
  // Please talk to your SSO adminsitrators about which format you should be using.
  'unique_id'               => 'http://schemas.xmlsoap.org/ws/2005/05/identity/claims/upn', 
  'user_name'               => 'uid',
  'mail_attr'               => 'mail',
  'sync' => [
    'mail'      => FALSE,
    'user_name' => FALSE,
  ],
];

If I commented out this whole block of code in my setings.php file then I can login to my dev.mysite.com/user drupal site. One other thing I'm not clear is, do I "Check Activate authentication via SimpleSAMLphp option" first then copied the code snippet to my settings.php file and push to Acquia or the other way around?

Any help is much appreciated.

Solution

It seems that update to version 8.x-3.0-rc2 resolves the error above. However, looks like it introduces another issues, "This site can't be reached" and redirected the site to port 80 instead.