Memcached – Unable to load the "SymfonyBridgeTwigFormTwigRenderer" runtime

I’ve been working through various hurtles toward migrating an existing Symfony3 web app from a local LAMP host up to Goggle’s App Engine Standard PHP Environment. I’ve taken hints from the appengine-symfony-starter-project to use Memcache for sessions, pre-cache the application code, and connect to a Cloud SQL MySQL database. In have the core pieces working fine.

I added a login form at /login using a TWIG template with a form in it that goes like so:

<form class="form" action="{{ path('security_login') }}" method="post">
  <input type="hidden" name="_csrf_token" value="{{ csrf_token('authenticate') }}" />
  <div class="form-group">
    <input class="form-control" id="email" type="text" name="_email" placeholder="Email" value="{{ last_username }}"/>
  </div>
  <div class="form-group">
    <input class="form-control" id="password" type="password" name="_password" placeholder="Password"/>
  </div>
  <div class="form-group">
    <input type="checkbox" id="remember_me" name="_remember_me" checked>
    <label for "remember_me">Remember Me</label>
  </div>
  <div class="form-group">
    <button class="btn btn-round btn-b">Login</button>
  </div>
  <div class="form-group"><a href="{{ path('security_reset') }}">Forgot Password?</a></div>
</form>

This works fine in the local DEV and TEST environments but when I push this up to GAE, I’m getting the error below:

Unable to load the "SymfonyBridgeTwigFormTwigRenderer" runtime.

I’ve been digging for a couple hours now and haven’t found a hit as to what’s wrong here. Anyone have any ideas where to look?

Thanks in advance,