503 Service Unavailable on /checkout/onepage/saveOrder/ and redirect to Cart

I am getting redirected to Cart after selecting shipping method. In the console log I saw checkout/onepage/saveShippingMethod/ 503 (Service Unavailable) and I don’t see anything in the error.log.

I tried the solution

Magento onepage checkout saveOrder 302 redirect

but the solution provided is already implemented.

I am using Magento 1.9.