I believe the problem is more on the polyfill side, but just to keep you in the loop :
Since Symfony 2.8.3 the symfony/polyfill-apcu is required, and if the system doesnt have APCIterator or APCUIterator, Stash trigger a fatal error when calling class_exists('\APCUIterator'); since the class is known by Composer's autoloader but can't be instanciate as it extend \APCIterator that doesnt exist on all system.
symfony/polyfill#39