The error is related to form CSRF protection, the error message comes from core form validation.

To avoid the error, it's possible to remove that CSRF token validation check in `hook_form_alter` e.g:

    function my_module_form_alter(&$form, &$form_state, $id) {
      if (strpos($id, 'webform') === 0) {
        // Bypass per user CSRF token check and allow cache.
        unset($form['#token']);
      }
    }

The caveat is, the form will no longer have default CSRF protection.

Alternatively, you might be able to detect the user has logged in (e.g custom [local storage set event][1]) and reload the original page to refresh it as is.


  [1]: https://developer.mozilla.org/en-US/docs/Web/API/Document/defaultView/storage_event