Anonymous users receive error 'LogicException: The database connection is not serializable.'

I noticed that if i comment 1 line at PageCache.php, then it works fine

    // Only allow page caching on master request.
    //if ($type === static::MASTER_REQUEST && $this->requestPolicy->check($request) === RequestPolicyInterface::ALLOW) {
      //$response = $this->lookup($request, $type, $catch);
    //}
    //else {
      $response = $this->pass($request, $type, $catch);
    //}

But i have no clue what lies behind all this (and why page caching is used at all when we generate captcha image)..

I'm using Drupal 8 beta10. I see no page_cache module in previous beta i have installed. CAPTCHA works well there. So something is definitely changed in beta10.

Had to disable "Internal page cache" module added to beta10 while it is not fixed.

Even using this (hack) currently to keep other caching functionality for anonymous users:

// core\modules\page_cache\src\StackMiddleware\PageCache.php

  /**
   * {@inheritdoc}
   */
  public function handle(Request $request, $type = self::MASTER_REQUEST, $catch = TRUE) {
    // Only allow page caching on master request.          
    if ($type === static::MASTER_REQUEST && $this->requestPolicy->check($request) === RequestPolicyInterface::ALLOW
        && strpos($request->getRequestUri(), 'image-captcha-generate') === false) {
      $response = $this->lookup($request, $type, $catch);
    }
    else {
      $response = $this->pass($request, $type, $catch);
    }

    return $response;
  }

Another nightmare i face now is that i'm using image captcha in the block, so i switched off block caching for that particular block. Same image is generated over and over it probably means it takes image from cache even if block caching is disabled. WTF drupal ... :(

P.S. with "Internal page cache" module disabled everything works fine.