I also want to add that you check your server configuration to make sure you're not giving special treatment to Google's block of IP addresses. Make sure your server firewall isn't configured to block Google's IP addresses.
Also, make sure your .htaccess as well as other configuration files do not contain entries that direct the system to give a bad response to Google's IP addresses.
Another possibility is that your server configuration is set so that only certain browsers or devices are allowed access to the website and Google isn't one of them.
When you make changes to the server configuration, use the "fetch as google" tool in Google webmaster tools to make sure Google can access your content. Also, run it through other Google tools such as page-speed insights.
Another thing that can help is for you to check all access logs, especially right after using the "fetch as google" tool to see what IP address and user agent string google uses so that you can redirect the correct bot to the actual content instead of to an error page.