This error means that your server takes too long to send a response. I just tried it with curl and it actually takes quite a long time:
After trying it several times, I was able to get a card preview to work, so it seems it’s set up correctly but your server is just very slow sometimes.
You might want to look into some caching plugins or Varnish caching proxy to get a faster response time. Ideally it should be less than 2 seconds.