When using the Drupal 8.3 Bootstrap theme if you enable "Rebuild the theme registry on every page load" in /admin/config/development/devel your page may fail to load with the error:

The website encountered an unexpected error. Please try again later.

If you check the watchdog logs the following error appears:

Error: Class 'Drupal\bootstrap\Bootstrap' not found in include_once() (line 30 of web/themes/contrib/bootstrap/bootstrap.theme)

Sample code to convert Drupal 7 usage of ajax_command_replace and render do the Drupal 8 equivalent when doing form AJAX callbacks.