Flag Plus install fails for some combinations of PHP >= 5.4 and old X Autoload

It was found that the class detection failed when migrating from one server to another with a higher PHP version, giving this message:

Fatal error: Class 'Drupal\flagplus\banner\BannerSchema' not found in  .../flagplus.install on line 17

The solution is known, upgrade X Autoload to at least [xautoload-7.x-5.2].