The website is under heavy load + ROR
You have too many requests in queue. Since version 4.0.15 there is a limit which is 100 by default. Here is a short excerpt from http://blog.phusion.nl/2013/09/06/phusion-passenger-4-0-16-released/ which says:
Phusion Passenger now displays an error message to clients if too many requests are queued up, instead of letting them wait. This much improves quality of service. By default, "too many" is 100. You may customize this with
PassengerMaxRequestQueueSize(Apache) orpassenger_max_request_queue_size(Nginx).
Have a look at the user guide about this: http://www.modrails.com/documentation/Users%20guide%20Apache.html#PassengerMaxRequestQueueSize
You could try increasing it or setting it to 0in order to disable it.
EDIT
You should also check your logs to see whether there are requests which take too long. Maybe you have some processes in your code that take too long. I prefer using NewRelic for monitoring those things.