There seems to be a random error that occurs which maybe due to the load from queries but customers are able to fill their cart and utilize all other functions of the website with the exception that they try to checkout and they get the following error.

Proxy Error
The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request GET /store/index.php.

Reason: Document contains no data

My business is suffering and I would like to know if anyone has received the same error.

This error does not occur at times but commonly occurs at random. I have tried to see if there is any commonality and I can not find one. My web designer has checked the code and does not believe this to be a written code error. Please help.